Dual boot Yosemite and Big Sur (HFS, APFS and other potential challenges)

Question

I'd like to use a clean install of the latest Big Sur (11.4 or 11.5) on a machine that currently has Yosemite installed. I'd like to keep using Yosemite due to some legacy apps/licenses.

• Ideally I'd like to edit the same files from both Yosemite and Big Sur
• I've also considered accessing data and installing/running an app on a different partition to the one with the OS
• However, I'm realising that there could be challenges due to the HFS and APFS formatting

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I have Yosemite (10.10.3) installed on the Macintosh HD partition. I have two other partitions which I use as data storage (e.g. Word, Excel files).

P1) Macintosh HD (480GB, Yosemite already installed)
P2) 2 - 430GB
P3) 5 - 90gb

Scenario (i):

P1) Macintosh HD (480GB, Yosemite already installed)
P2) 2 - 430GB (Big Sur would be installed)
P3) 5 - 90gb

Scenario (ii):

P1) Macintosh HD (480GB, Yosemite already installed)
P2) 2 - 430GB 
P3) 5 - 90gb (Big Sur would be installed)

All three partitions according the disk util are:

   File System Personality:  Journaled HFS+
   Type (Bundle):            hfs

Question that arises:

1. With reference to the two scenarios above, is it possible to access (both read & write) files (e.g. a Word document) and or run apps stored on a different partition than the one with the running OS?...Or will the partition (with the other OS installed) no longer be visible?

A.) If yes, how does this work if one of the drives is JHFS+ and the other is APFS?

B.) If not, would it be better to store data on a partition that doesn't have any OS installed (used almost like an external SDD except that it's internal)?

C.) Would there be any issues from installing apps not on the same partition as the one with OS-installed? E.g. app files that need to be located in the system, caches, application support and library folders

Machine: MacBook Pro (Retina, 15-inch, Mid 2015) 1TB SSD (3 Partitions):

/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            479.5 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
   4:                  Apple_HFS 2 - 430GB               430.3 GB   disk0s4
   5:                  Apple_HFS 5 - 90gb                89.7 GB    disk0s5

I've made an attempt to answer my question below but I'm still unclear on a few points (I've added my research with links there).

Answer 1

Here's my attempt to answer but still not very clear on a few things.

For maximum usability of the dual-boot with minimum performance/bug risk, I'm considering going with scenario (ii) with partition 3 formatted as APFS.

This way, I'd be able to store more data (430GB instead of 90GB) that is visible (and accessible, ie. read & write) when running Yosemite and also when running Big Sur:

• Yosemite can see P2 but not P3
• Big Sur can see both P1 and P2

In this case, I'd assume 'seeing' is good enough for read & write, if I'm not mistaken.

Another option would've been to have all partitions on APFS but apparently Yosemite doesn't work on APFS? Alternatively put everything in HFS+ but risk not being able to update Big Sur?

Links I've had a look at:

1. Dual boot dual partition and clean install of High Sierra [@klanomath & @Chains here discuss using CLI to install Big Sur in HFS+ format]
2. 10.13 to Big Sur - HDD APFS System volume and HFS+ User Space [@bmike seems to discourage Big Sur on HFS+...suggesting potential bugs and or performance/setup issues, if I understood correctly]
3. https://www.reddit.com/r/MacOS/comments/odc717/dual_partition_boot_disk_with_yosemite_and_big_sur/ [I think what's explained here is even if APFS and HFS+ partitions are on the same SSD each partition won't be visible to each other]
4. https://support.apple.com/en-us/HT208891 [having two APFS volumes on the same partition?]
5. https://www.macworld.com/article/234913/macos-big-sur-supports-time-machine-on-apfs-formatted-drives-but-there-are-a-few-catches.html [Time Machine can't use APFS smoothly?]
6. https://mrmacintosh.com/macos-big-sur-full-installer-database-download-directly-from-apple/
7. https://www.youtube.com/watch?v=zHfJ0uHgktA [2 Easy Ways to Convert HFS/HFS+ to APFS Without Losing Data] - I don't think this is applicable to Yosemite.
8. https://discussions.apple.com/thread/8570503 ["If the Mojave partition is formatted as APFS, Yosemite won't see it." and "Avoid placing Mojave on hfs although possible: any minimum upgrade to Mojave requires it to be on an Apfs volume." — I assume these apply to Big Sur too]

Answer 2

I somehow ended up here as I am researching re-building my MacBook Pro (2015) with a pre-APFS OS (Snow Leopard) and a post-APFS OS (most likely the latest one possible).

Over the last few years, I have followed a different approach on building my Macs and I just wanted to mention that here. What I basically do is to separate OS from user files. At the default setup, macOS creates one partition where it throws the OS and also the user home folders and sub-folders. What I do is to generate one partition where I install the OS and another partitions for user home folders (and their sub-folders). I also created a 3rd partition for globally sharable files (e.g. Music, Films, e-books etc). The last 2 partitions are merely data storage and have no OS on them. The system partitions can then be serialised with as many OS version you may want to install.

Now, the key idea in all of this is that only ONE OS will be your main OS of choice. For example, I will use Monterey as my main OS, which will have the majority of my user files, applications, iCloud etc. The other OSs will probably be minor in comparison and hence they can have their own user folders in the same partition as the OS or mapped to some NAS for transparency. The Advanced Options for Users in System Preferences allows the change of home folder to be done really easily. There is also the Directory Utility app, that can do similar things (and probably a lot more I am not even aware of).

This seems to work OK inasmuch as the separation of OS and user files, which eliminates the complication of accessing one OS partition from another one (e.g. accessing the Monterey partition from Snow Leopard - we now know it's not going to work).

Anyway, just my 2p on the matter. I am a rather basic Mac user, nothing crazy but do like "tinkering" with the default setup to make it work for me.