As a bass player, I regularly come across performances that require me to double on instruments. Typical combination would be Electric Bass Guitar and Upright Bass or Electric Upright Bass. Sometimes i also encounter instances where i need a fretted and a fretless bass; and in some occassions i need a synth keyboard bass for those electronic dance tunes or old-school disco / funk gigs aside from my standard electric bass guitar.
The most critical piece of equipment for me as a doubler is my instrument selector switch / pedal. This is where both instruments are plugged in and sent to one amp; choosing instruments via toggle switch. My current switch is the Radial Engineering Bigshot I/O. The switch has a small volume knob which balances the volume between the two instruments. Another feature that i find very important is that it does not require any power source to use. This allows me to keep it plugged-in the whole night without worrying about the battery.