Bassman amps aside you would think with the modern high wattage circuits that would be a very reasonable request. There are some really cheap amps out there that say bass/guitar on them but I believe the bass and the guitar would probably sound the same "crappy". Outside some simple switchable input and output circuitry the big hurdle would be the speakers. Without loading the cab with different speakers for each you would be forced to use a high power full range speaker and would then lose some very fundamental characteristics. But I guess for practice sake you would mind that too much. What do you get when you mix a "Roland Cube" and "Line6 LowDown 10"? I'd say very doable!!