You can't buy enough hardware for that, so we'll have to resort to internet trickery.
Each performer is at home and logs into the same Skype session (free). They perform to each other using headphones and the built-in microphone on each laptop (free), but the trick is they record their own high-quality voice, each on his own computer, during the performance. You don't use the bubbly, skipping Skype voice.
Then, at the end of the show. Each performer sends his high quality voice to one person who mixes it all together in sync to the final show using Audacity (free). Include music or other effects at this step. Don't send the voices in MP3 because you can't do good production in MP3. Use FLAC or other high quality compression.
If the voices are too big for email (I think Yahoo Mail limit is 25MB) then you'll need to cut each performance up into smaller files and email each one (free).
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This process substitutes time and hard work for money.
If you don't do something like this, the multi-microphone mixer will run you around $100 - $150 and each microphone will go for $50 to 100, plus headphones, amplifiers, etc, etc. and you'll still have to do music production in post -- after the show.
This mixer will do four microphones. It's the one we use at work.
http://www.sweetwater.com/store/detail/PV6
http://www.kozco.com/tech/MicTests/studioLayout.jpg
Koz