The best you can do with two USB microphones is Aggregate Device which jams them together into one. This works best with two identical microphones. This is easy in Macs and Linux, but the jury’s out on Windows.
USB microphones are aggressively non-expandable. They’re one trick ponys. You can use one in Overdubbing/Multitrack where you build up a song instrument by instrument and you can use two microphones and two computers. Combine the two tracks, but even there you can get into trouble. Since you have two different digitizers, the song’s parts can drift out of sync as the performance goes on.