i did this a while ago. put headphones out through the computer to listen to the metronome, and put those headphones on the guitarist. plug him into an amp that has a headphone jack out, and put those headphones on the drummer. drummer will play to the guitar (or bass, whichever is easiest to follow) and there will be no other bleeding through of instruments. after you have the drums laid down, its just a matter of hooking the peices up.