You should look for a stereo speaker, also called a ‘4 wire’ speaker.  Basically, both the left and right channels are in one single speaker that’s really designed for situations like this.   If your existing speakers and the new speaker are all 8 ohm, then you can just connect everything directly to the amp.  That will mean that both bedroom and bathroom speakers will play at the same volume.  You can get an impedance matching switch if the speakers are not 8 ohm, with volume control if you want that.

From my experience, this sounds good in theory, but the lack of different sources and volumes in the two rooms will get old.  It’s cheaper and more functional to just add a Roam, or pair, to the bathroom.  They are small enough to take very little space, waterproof, and sound as good as any ceiling speaker you would get for the space.

Thanks Danny.  The hole in the ceiling is already cut and the wire ran to the amp so I would prefer to put a speaker in the ceiling, but if not for that, I think you would be right about going with a Roam.  In my quick search I found a Polk RC6 that is 8ohm to match the Sonance by Sonos architectural in-ceiling speakers.  I suppose another option although more expensive, would be just to put it on its own amp.

I believe that’s the exact speaker I used for my bath in a previous home.  It sounded great.