How did you go about listening to the first track while recording the second? My hunch is that you listened through speakers (rather than headphones) and the microphone picked up audio from the first track while recording the second (and also introduced a delay because of the out-and-back loop that the playback of the first track is making) . To avoid this, you need to use headphones to hear the first track while recording the second.

i should have clarified: both were recorded independently through my sound card's input from direct outputs from each instrument (electronic drum set, direct out amp)

Sounds like the computer can't keep up. CPU overload, that happens sometimes if I try and track something while running too many programs at the same time. Of course, when using your computer's built in soundcard to record real time audio, it doesn't need too many excuses to fail unfortunately. They weren't designed for what you (and admittedly many many others) are using them for.

i figured it was probably the sound card. it's an old Sound Blaster Live. pretty sure all other hardware on my computer can handle the workload though.

will buying a new consumer-level sound card fix this?

thanks