@egnos
Sorry for confusing.
Logitech Bluetooth Speaker Adapter might be one of that kind.
But as you know, Multi-pairing doesn’t mean MIXING two output at same time. You should hold one source to use another different source. (And I’m not sure it will be returned magically after occupation from one source is finished.) Many speakers insist switching could be automatically, but I’ve never seen those things as they said still yet.
multi-channel BT mixer connected to one speaker could do, but It might not be your wants.