O.K. This might sound like total science fiction, but I'm going to put it out there and see where it goes!
Let's say that I have a split bass and piano with a one octave overlap. Let's also assume that anything I play using my left hand is bass and anything I play with my right hand is piano. How would my MODX know which hand I'm using when I'm playing in the one octave overlap area?
Well, one idea would be to work with the velocity so anything played 64 and under will sound the bass, and anything with a velocity of 65 and over will sound the piano. Of course, I would probably introduce some velocity offset so the bass isn't too quiet, but it's certainly a start.
Any other ideas?