Use the notes held on one (silent) input and uses them to change the channel of the same note numbers when played on the main input.

details here: https://tinyurl.com/2keyboardRechannel

Could be adapted for one keyboard (or buttons on a controller) quite easily.

From an idea by MIDIman:
“This way, chords held on keyboard B (but not heard), could then cause corresponding notes from the keyboard A performance to be redirected into radically different timbres. This would open up some exciting and unpredictable interactive musical possibilities.”

**see notes for why this is two patches in one**