Nice question...
To plug both inputs into one amp, you would need to modify the guitar, by switching the "black" and "white" wires on one of the p/u's... don't bother...
try this: BIGSHOT ABY
Use the bottom example... This might work well, it has a 180deg flip switch, and it's passive...
Further to that, a stereo FX pedal of some sort may do the trick as well... Type "stereo guitar pedal" into search at musician's friend for some eamples.