Here's another thought - if you are using an amp that can handle a 4 ohm load (something that receivers rarely can do well but separate amps handle routinely) and desire a way to operate each set of speakers somewhat separately, you could put a speaker selector switch between the amp and the speakers so that you could toggle between bedroom, bathroom, or both. When both were on, the amp would see that 4 ohm load, but that'd be true anyway.