I've got a Bose SoundLink Mini portable Bluetooth speaker that I use in multiple ways, with multiple "partners." In my shop I pair it with my phone to listen to podcasts. While doing dishes, I pair it to an iPad placed behind the sink to watch TV. At the dining table, my wife and I will occasionally pair it to one of her devices to watch something.
Switching the pairing is never instantaneous, and can be a little fiddly. Spezi, a portable speaker concept by German industrial designer Silvio Rebholz and the Yamaha Design Lab, tackles this issue with a wonderful, tactile interface: Multiple telescoping antenna-like controls, one for each paired device, that you pull up or push down.
Pushing one all the way down silences that device; pulling one up increases the volume, "which also gives visual feedback on its volume," Rebholz writes.
Additionally, "by overlaying channels, transitions in between songs from different devices become possible and turn the speaker into a music mixer." I'd be happy to just switch between devices in an immediate and physical way.