If memory serves correctly:
Midi 0: stores all channels in one track, as in if you open a MIDI0 file in a sequenser, all the notes would be on track 1. The notes refers channels and instrument used to play that note (and of course volume, pan, velocity, etc).
Midi 1: stores one channel on one track, as in if you open a MIDI1 file in a sequenser, track 1 would use channel 1 etc.