You could set up for Overdubbing, and conveniently enough, I wrote some of the instructions to do that.
You should probably start with the one on the bottom – Fake Overdubbing. That one’s the least crazy or expensive.
http://www.kozco.com/tech/audacity/overdubbing/overdubbing.html
That one has you using your built-in microphone and you would substitute your own microphone, but the principal is the same.
Koz