BT Classic HFP and A2DP supports multiple simultaneous connections at a time (i.e, 2, AFAIK). However, this does not mean you can actually hear multiple sources at a time. The way it usually works is that if one device is quiet and the other has audio, it switches to the one that has audio. What it does when both have audio I don’t know for sure; it probably gives precedence to the one it was on or picks one randomly if it’s just starting up, and then does it’s switching in the normal way.
For instance, my Bose headphones support at least a half-dozen different pairings, but they will only connect to two devices at a time. They connect to my TV BT transmitter (that I used to use before the Marvels) and to my phone, for instance. And the Bose headphones tell you which two (out of your bigger pairing list) that they’re currently connected to. I’ll be listening to TV, and then if a phone call comes over, it immediately switches to the phone.
On the other end, a BT transmitter can connect to 2 receiver devices at a time. Even with HFP. But again, it’s an either-or connection. Case in point, when I get into the car, my Marvels stay connected to the phone but the phone also connects to the car BT. When a call comes in, the phone rings itself, but simultaneously rings through the car audio system and rings in my Marvels. When I answer the, it gives precedence to the car BT system, but can be switched instantly in the phone’s call progress screen.
Likewise, my phone’s A2DP stays connected with my Marvels but also connects to the car BT audio, and gives precedence to playing through the car’s sound system.
Hope this helps.