I doubt you could ever get the transports on that class of machine to even come very close to being usably in sync(they drift quite a bit,but what you COULD do is record 4 tracks on the 1st machine and then reocord those onto 2 tracks of the second machine(through the mixer),which would now leave you 2 more tracks to record onto on the 2nd machine.I think you'll find that if you go any further than that you hit the diminishing returns zone where the sound gets so mushy that there's little point in trying to add more parts.There's an article in a recent Tascam brochure about some guy in I think Michigan who is getting Radio Airplay with a tape he made on a cassette 4 track! Cheers!
