It's possible. You'll pretty much have to send .wav files back and forth, and those get big, and transfers get slow, but it can definitely be done. The number one most important thing is going to be defining and sticking to the start time on each set of files. If you send him a mix, and he records something where he doesn't start playing til halfway through, the file he sends you back still has to start at the very beginning even if that means a whole lot of silence and "wasted" drive space. Otherwise you'll dick around forever and probably never get it perfectly lined where it was when he recorded it.