The actual answer is that the premise is wrong. Proper async USB DAC’s don’t adopt to the input, they dictate the timing, not the other way around. It is the OS that isthe slave in this case, and sends audio in when requested. The only thing the OS does is set the sample rate the DAC should use.
Now with multiple devices, you have multiple of those requesting audio samples, and this will not be in sync. So to keep them in sync, some kind of ASRC needs to be employed to keep it working. Easiest would be to use one of them as master timer, and then change the rest accordingly. In principal, the individual samples rates would not even matter anymore in this case.
What MacOS actually does, I don’t know..
Edit: seems like I was quite close: https://support.apple.com/en-us/HT202000
You can select one of the devices as master and select an option for clock drift correction (aka ASRC)
I heard people on diyaudio complaining about shifting center image when using the aggregate device functionality, I never saw it backed up by any data so not sure how real it was.
As I have a few identical USB DACs and a Mac I will add it to my testing list and post the results. Mac ASRC is pretty good so I would expect the performance to be good as long as it is actually able to keep sync.
Michael