Using multiple Bluetooth units

My wife and I each have a Prius, and we both have iPhones. Sometimes I also use an Android phone (Google Nexus 5).
In my car (2011), I can use my iPhone both for phone calls and as a media source. I can switch to the other phone by using the "Connect" button on the phone screen. But even if I attach another phone for the phone function, the media function seems to be locked to the first phone.
I have also tried to use the Android phone as a USB device, but even though the phone says "connected as a media device", the USB button on the audio screen remains greyed out/disabled.

Where do I find a detailed description on how this is supposed to work?