@forteller Yeah, it really seems like existing devices should do this! I don't know if it's a matter of nobody implementing it in software or a matter of actual hardware limitations to do with audio processing or number of bluetooth devices that can connect & how data is routed to them... but it really seems like it ought to be possible!
It's been a while since I read about that dongle trick and I don't have the link any more. Probably came from a cycling or motorcycle forum and I have no idea how to find it again, though I would have initially found it via some combination of web search & following links...
And now that I'm thinking of it, I realize I was misremembering the way it worked! I believe they actually had wired headphones plugged into some kind of standalone bluetooth audio device?
(I tried walkie talkies and the audio quality was appalling. I also tried some "bluetooth walkie talkie" apps on phones, which never really worked right.)