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Is it possible to connect two external 4K60 monitors to a M1 Pro macbook using a single Thunderbolt 4 port, more specifically using a dock?

I have researched this and am getting conflicting answers. The official docs aren't helpful, as they don't specify this. I found this related question on Apple forums, and some answers there say it's possible.

But then I asked the specific question there myself, and got an opposite answer.

I called Apple, and the rep also said that this is not possible even with the M1 Max (only one monitor per port), but I am not 100% confident in their knowledge.

People here are sharing their personal experiences and confirming that it does work with specific setups.

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  • I see a bounty placed for someone to provide an "authoritative source" for an answer. It would be helpful to know the criteria for an authoritative source. Apple documents show dual displays on a single Thunderbolt dock is not supported. However people have shown it working by booting into Windows or Linux, using DisplayLink docks, or using an eGPU. What source would be more "authoritative" than Apple? The answer is that it is possible, but not supported by Apple, that means getting it to work will be something of a "hack" and prone to difficulties.
    – MacGuffin
    Dec 16, 2021 at 22:01
  • > Apple documents show dual displays on a single Thunderbolt dock is not supported. could you link these, please? You say people are getting it to work with workarounds, but I've seen reports of it working out of the box without DisplayLink (supposedly). So I want a generalized answer that can tell me for sure which docks support it and which don't, what the limitations of M1 Pro are, and how to tell if a dock supports it before buying. All the answers are anecdotal and conflicting so far.
    – gshpychka
    Dec 17, 2021 at 8:15
  • I wonder whether "Using docks or daisy-chaining devices doesn't increase the number of displays you can connect" only relates to the plain M1 models or whether the formatting is off and it's actually valid for all M1 models.
    – nohillside
    Dec 17, 2021 at 9:03
  • @gshpychka " could you link these, please? " I added links and some more explanation in an edit to my answer below. There is no one "generalized" answer on this working other than "it doesn't work". The people that did get it to work had to do so by loading an different OS, "hacking" the drivers to add MST, using a DisplayLink GPU (or other USB based GPU), or a Thunderbolt GPU. One possible explanation on people claiming it works is confusion over the goals. Two displays are easy enough if they are mirrored.
    – MacGuffin
    Dec 19, 2021 at 21:19
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    Claims made by random people on the internet need to be taken with some healthy amount of suspicion. I write that knowing that I am included in the population of "random people on the internet". I'm basing my claims on what Apple has listed as supported, and experience from many years in technical support that Apple is often "conservative" on what their hardware is capable of doing. I realize you want confirmation before you try yourself but at some point that's what it may have to come down to.
    – MacGuffin
    Dec 19, 2021 at 23:01

4 Answers 4

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This one will do it with one cable. Note that it does not use (or need) Thunderbolt 4 to do so:

VT4510 Dual Display 4K USB 3.0 & USB-C Docking Station with 100W Power Delivery

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  • True. But it does use DisplayLink and it's not Thunderbolt but USB-C.
    – Volsk
    Dec 16, 2021 at 7:50
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    @Volsk I know I may be overly pedantic but USB-C is not distinct from Thunderbolt (TB). USB-C is the physical connector, a connector that can carry either TB or USB 3.x protocols at the same time. USB4 allows for "superspeed" USB and TB on USB-C at the same time. TB can use USB-C or mini-DisplayPort for the physical connection. USB 3.x can use USB-C or USB-A on the host port. USB4 is a superset of both TB and USB 3.x on a USB-C port. The dock given in the answer is not TB and so does not technically answer the question but perhaps meets the intended goals.
    – MacGuffin
    Dec 16, 2021 at 21:53
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Is it possible to connect two external 4K60 monitors to a M1 Pro macbook using a single Thunderbolt 4 port, more specifically using a dock?

No, it's not possible. There are exceptions that I'll get into but generally no, that is not possible. This has been a long time complaint of Apple computers and it does not appear Apple is going to change this.

The main issue is Apple does not support MST, multi-stream transport, over DisplayPort. This means one port supports only one display.

Apple states on their website that computers with M1 and Thunderbolt 4 support only one display per port. It's in their support page on connecting displays: https://support.apple.com/en-us/HT202351 And on identifying ports: https://support.apple.com/en-us/HT201736

The exceptions to this are displays controlled by a separate GPU. Someone could plug in a dock with a DisplayPort output and USB, and then use a USB GPU like those made by DisplayLink to drive the second display. Some docks have DisplayLink GPUs built-in to them for multiple displays. Another means to get an external GPU is with a box that is clearly labeled an external GPU. These are docks with a primary function of adding additional graphics processing, though this processing is available to be used on an internal display as well as external displays. These eGPU docks will tend to have other ports like USB and Ethernet but some do not as their function is to be a GPU, not a general purpose dock, and so may lack such ports as they take valuable bandwidth.

If connecting two displays by one Thunderbolt cable is important to you then you are going to have to pay for it. There is no cheap resolution to this. Windows will support two displays on a single Thunderbolt port but you pay for it in a different way, lower video performance. Two 4K@60 displays on one Thunderbolt port is asking a lot, if it works at all then it's likely by "cheating" with compressed video. Maybe the math works out and no cheating required but it's getting real close to the limits of the cables.

DisplayLink and eGPU get by with less bandwidth on the cable to the computer because the video is still a stream of descriptive bits at that point. After processing into the full uncompressed video frame there's more data. After processing there is more bandwidth consumed on the cable to the display, one 4K@60Hz consumes more data than one DisplayPort channel can hold. Supporting two 4K displays means using all 4 "superspeed" channels in the USB-C cable for video, leaving no channels for Thunderbolt or USB 3.x, or multiplexing the DisplayPort video into the same "superspeed" channels, which is a feature not all displays and docks support. I speculate but it's likely safe to assume Apple didn't want to confuse users with the long list of technical requirements on what is needed to get two displays on one Thunderbolt port to work and simply says it is not supported. They "enforce" this limitation by not including MST support in their OS and drivers, without MST there's no multiplexing two DisplayPort streams on one cable.

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  • So to clarify - is this answer wrong or do those people who are reporting it working all have docks with DisplayLink and that's why it's working?
    – gshpychka
    Dec 17, 2021 at 10:34
  • >Apple states on their website that computers with M1 and Thunderbolt 4 support only one display per port I'm asking about M1 Pro/Max, though, and they don't say anything like that regarding them.
    – gshpychka
    Dec 19, 2021 at 22:08
  • @gshpychka " I'm asking about M1 Pro/Max, though, and they don't say anything like that regarding them. " That's not how I read it. I'll phrase my claim differently then, as Apple makes no mention of supporting more than one display per port the assumption is that this is not supported. If Apple supported two 4K displays on a single Thunderbolt 4 port then they'd have a document stating how to make it work. If there is a non-Apple product that offers two 4K displays on one port then you'd have to look for how to make this work from the people that make that product, not from Apple.
    – MacGuffin
    Dec 19, 2021 at 23:12
  • Addendum to emphasize an earlier point... The way to know if a dock is using DisplayLink to drive multiple displays is to do a little research on the dock. A dock that is using DisplayLink will almost certainly have the DisplayLink trademark on it. A dock using DisplayLink will need drivers to make it work, will have the device appear in System Information, will have DisplayLink mentioned in the technical specifications, and an app to configure the GPU should appear somewhere on the computer's boot drive. It should not be difficult to discover but also it may not be obvious from a glance.
    – MacGuffin
    Dec 19, 2021 at 23:27
  • That's not how I read it that's definitely what they meant, though - the original M1 only supports 1 external monitor, M1 Pro supports 2, M1 Max supports 4. They don't say anything about not supporting them on a single port, that's still up for interpretation. Another important thing that contradicts your answer is that Intel-based Macs do support multiple 4K screens on one port. And also the fact that the Apple rep told me that the M1 Pro/Max do as well, just not 4K.
    – gshpychka
    Dec 20, 2021 at 7:20
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Yes this is possible. I'm doing this with a Thunderbolt 3 dock. Specifically HP G2 Thunderbolt Dock G2. The crux is to connect one display to the DisplayPort and the other to a Thunderbolt port (provided your dock has one). Based on the specs listed on Apple’s website your MacBook is capable of running 2x 6K@60. The dock merely acts as Thunderbolt pass through. So you should be good.

This is inline with the statement in Apple's support article About the Thunderbolt ports on Mac:

Depending on your Mac, you may be able to connect multiple Thunderbolt devices to each other, then connect the chain of devices to the Thunderbolt port on your Mac. See Use one or more external displays with your Mac.

Dell has instructions for this specific setup (2x 4K@60Hz) with their WD19TB Dock:

Dell WD19TB Dock connections Dell also instructs to connect one display to the Thunderbolt port on the dock and the other to Displayport or HDMI.

Wirecutter mentions a similar setup in their review of Thunderbolt docks:

The second Thunderbolt port allows you to pass through data from Thunderbolt accessories. You can hook up two 4K monitors at 60 Hz, too,

OWC, known for its Mac accessories, also has a dock that can do this. On the page about this dock, they refer to an infographic on Thunderbolt.

My specs below for reference.

enter image description here

Apple M1 Pro:

  Chipset Model:    Apple M1 Pro
  Type: GPU
  Bus:  Built-In
  Total Number of Cores:    14
  Vendor:   Apple (0x106b)
  Metal Family: Supported, Metal GPUFamily Apple 7
  Displays:
DELL U2518D:
  Resolution:   2560 x 1440 (QHD/WQHD - Wide Quad High Definition)
  UI Looks like:    2560 x 1440 @ 60.00Hz
  Main Display: Yes
  Mirror:   Off
  Online:   Yes
  Rotation: Supported
  Automatically Adjust Brightness:  Yes
Color LCD:
  Display Type: Built-in Liquid Retina XDR Display
  Resolution:   3024 x 1964 Retina
  Mirror:   Off
  Online:   Yes
  Automatically Adjust Brightness:  No
  Connection Type:  Internal
DELL P2417H:
  Resolution:   1920 x 1080 (1080p FHD - Full High Definition)
  UI Looks like:    1920 x 1080 @ 60.00Hz
  Mirror:   Off
  Online:   Yes
  Rotation: Supported
  Automatically Adjust Brightness:  Yes
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  • Comments are not for extended discussion; this conversation has been moved to chat.
    – nohillside
    Dec 16, 2021 at 13:12
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M1 Mac can have only one native external display connected. However, it had been improved with M1 Pro Mac and M1 Max Mac systems(released after 10/2021. The new M1 Pro or M1 Max Mac systems can support up to 6 external monitors at the same time totally.

I had tested and proofed this new video supported capacity with the new M1 Pro/ M1 Max accordingly.

I added a Thunderbolt 4 HUB or Thunderbolt 4 dock to any of built-in Thunderbolt 4 ports on the Mac systems and the HUB/docks have up to 3 Thunderbolt ports or video ports on it.

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  • This does not answer the question of whether they support 2 4K displays on one port.
    – gshpychka
    Jan 7 at 14:21

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