I dunno, I would record the game play with your friend talking.
Then record your seperate audio with Audacity.
You should have some sort of TICK that helps with syncing audio cause that can suck sometimes.
So like, if you did "One, Two, One, Two, Three" and like moved something on the recorded footage before actually playing the game but recording the game and audio from the game and YOUR audio it helps out a lot with the finished product.
Wow this was really hard to talk about. Hope that helped!