Guess I must have missed something along the pike. I've never seen a single potentiometer hooked to more than a single device. When two instruments are connected through the same input, the output inpedence of their respective transformers is halved because they become parallel, resistive devices. That's what pre-amps and mixers have separate channels, blocking diodes and lots of other directional components that prevent impedience changes and problems with polarity differences between various types of equipment. Personally, if they were my keyboards, I would go with either a 4-gang pot in the foot-volume control and dual controls physically hooked together.

Good Luck,

Gary