Roger,
why cant you playback two recorded sessions at the same time?
Because the TIU is playing it back, not the remote, and the TIU can't be in two places at once.
I haven't tried this myself, however, try this. Play back a session from one TIU with one remote and a second session in a different TIU with a different remote. You'll have to be sure that both TIUs were not in Super TIU mode when the sessions were recorded and also when they're played back.