I want to record our Barbershop quartet, and I have 2 USB Mics. How can I get Audacity, or Garage Band, or any other software to recognize both mics and use both of them to record at the same time to make a stereo recording?
To do what you are wanting to do might not be possible. I believe that you will be needing a mixer, and definitely a microphone pre-amp. You will not be able to connect USB microphones to that kind of a setup . To connect the mixer to your Mac, you will also need either a iMic, or an iConnex as an USB interface connection. Neither one of these products work very well, but is better than nothing.
Contact your local Apple store and for way too much money, they will happily set you up.
OR you can record a single track with Audacity and then adjust the settings to playback in stereo.