Ya Know what? This question really got me ta thinking! (That can be dangerous).
IF:
1. You have two Identical 4 tracks (same model manufacturer etc.)
2. You can handle a soldering gun, phillips
screwdriver and some other basic hand tools.
3. You are willing to void your warranties on both machines.
THEN:
1. You could open up each unit carefully,
locate the wires going to the drive motors
and just use one set of wires to attach both
motors. (Tape off the bare ends from the slave machine).
2. Theoreticly both cassettes should stop/start/pause exactly at the same time.
3. This would enable you to record in sync
across all 8 tracks.
BUT: Don't get mad if it doesn't work perfectly. (You would have to keep a matched set of tapes together, and be careful not to let one of them turn with respect to the other).
If it fails completely, just re-connect the original wires to the 2nd unit, and go buy an 8 track!
Almost Sincerely;
Dom Franco