Just a quick follow up to those who'd kindly provided advice.

I had previously used this for 2 channels of harmony and found that it worked really well using Cakewalk (very easy to selct the appropriate bars and/or to loop the same selection ad infinitum). I quickly discovered though that I needed a PC accompanist, as in 'let's do the fugue from bar 324 to 368'.

Proof of concept of the 4 channel MIDI approach - using 2 soundcard audio outputs and 2 MIDI keyboard outputs- was moderately easy. 'Twas a bit rusty on the software side, but I eventually got it right.

The musical project fell very flat before I could put my rehearsal technique into place, but I'm moderately convinced that it can reduce the lenght of each rehearsal, and (maybe) reduce the number of rehearsals. At the risk of sounding uinfair, most rehearsals spend more time on the weaker sections, while the other groups sit around like cold bottles of pee. At least this way everyone gets a chance to rehearse MOST of the time.

Thanks again, somewhat belatedly,
John