I do a similar thing, with a Yamaha electric Grand. I use a splitter for the 1/4" in. I use a Hammond xK-1 on top of the piano, and use a nylon string Giannini with preamp, thru a sans-amp, into the other input. Actually, a guitar with a preamp sounds pretty good just straight. An electric, say, a 335 with humbuckers needs the sans amp to make the associated input adjustment.

A sans amp (either bass or guitar version) works OK directly into a PA head.

Russ