Cappuccino...Using Garageband I can't really use two sources simultaneously, so instead I've taken to using a Blue Yeti microphone set on stereo mode which at least gives me what amounts to a stereo pair of mics into a single channel. Great for almost every instrument except voice (so far). The odd time I have to record two players at the same time I set it to the figure of 8 pattern and do a number of scratch tracks to determine the approximate balance (and if there ultimately proves to be an imbalance either re-do it or double one or both separately). Interestingly, some judicial use of EQ can give the impression of volume balance on the same track as long as it's in a register that isn't entirely shared like violin and guitar...but one has to really narrow the frequency.

Peace.