So if you set Audacity to use a USB mic, then shut down Audacity and unplug the USB mic, and then restart Audacity, what, in your opinion "should" Audacity do?
Audacity can't currently record from two USB mics at the same time.
With conventional mics (non-USB) connected via a 2 channel audio interface, you can record as a stereo track, and then split the stereo track into 2 mono tracks.