This idea was more about a product I'm looking for. And when I actually looked for it, briefly, I didn't find it as expected. So I'm listing it here now until I run across it, make a serious effort to find it, or someone tells me about it.
The situation was I visiting for a bit by a friend and thought a good time to play a game on the mobile phone. My friend wanted to watch and listen along, but I was already busy with earbuds, as those are almost critical for some games, especially the stereo effect.
What I needed at that point then was a bluetooth splitter that would connect bluetooth to my phone as source, and then play to my friend's speaker and my earbuds.
When I looked briefly I only found cabled sound source input to multiple bluetooth output.