RW, here's an inexpensive but not very sexy solution: I have this old Midi-Thru box made by Casio - yep... Casio, those movers and shakers in the synth world. It has two midi INs (A & B) with multiple outs (obviously). You could bring your two Master keyboards into the A&B, and go out to your preferred (slave) sound generator, and then simply flip a switch to designate which keyboard will control the slave - but not both at the same time (Hmmm, a cacophany of SysEx messages could be problematic anyway).
The "un sexy" part is that you wouldn't want to be flipping switches in the middle of a song - but the solution will only cost you about $20 bucks if memory serves.
There... that's all I know about everything!
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Andrew
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Andrew