It used to be that two USB audio interfaces/mics would require you to create an aggregate device in order to use them both at the same time with OS X. As this was still the case in early 2014 (see the Macworld article, below), it probably is still the case today.
However, there are differing reports when it comes to identical USB devices.
Back in 2011, an audacity user was testing this specifically and reported:
So, far from being the outlier, Aggregate Device is required any
time you plug in two identical USB sound devices.
But in 2013, Blue Microphones themselves are reported to have said:
Because of the nature of USB devices, you won’t be able to use two of
the same USB microphones at once. Since they have similar digital
thumb prints your computer won’t be able to distinguish the devices as
separate devices. With that being said, you can use two different USB
devices/microphones at once with no problem. As far as being able to
use it within a specific program you would have to find out if it
allows multiple USB connections [...]
So, without further information, your best bet would be to try to create an aggregate device from both mics, and then test with an application which can record multiple channels (which I assume you already have since you want to do this in the first place).
This Macworld article gives details:
Plug both USB microphones into your Mac, venture to the Utilities
folder (/Applications/Utilities) and launch Audio MIDI Setup. In the
bottom left corner of the resulting window click the plus (+) button
and choose Create Aggregate Device from the menu that appears.
To the right you'll see a list of all the audio devices attached to
your Mac, including those two mics. Tick the Use checkboxes for the
two microphones (if any of them have multiple entries, look for the
one that has a 1 in the In column). As you do so, subdevices entries
will appear above the list of audio devices. This tells you which mic
is using input channel 1 and which is using input channel 2. If you
like, click on the name Aggregate Device in the column to the left and
change it to something more intuitive such as “Two Mic Setup.”
If you are able to do that, then the resulting device should be both mics, one on each channel, so they can be recorded separately (or, potentially, recorded as stereo and separated later).
Of course, if only one of the mics appears in Audio MIDI Setup then it's possible you won't be offered the option of selecting them both when creating an aggregate device.
Sadly, this post suggests it will not be possible but, if you contact Blue, they should be able to 'reprogram' one of your mics so you can get this working as an aggregate device:
Bue Yetis have identical digitial footprints and therefore no matter
how many you plug in, they will be recognized as a single device. I
contacted Blue and ultimately I was able to send one of my Yetis to
them for reprogramming and now my Mac will recognize it along with
another Yeti in order to create an aggregate device.
Fair warning, this process took over 2 weeks and I had to pay out of pocket
shipping, they paid FedEx ground (signature required) back to me.