Well, I was mostly in your same page, it just doesn't work (didn't work actually).
It happens that dual audio was working all the time in my S22 Ultra, the problem was actually the device (the Bluetooth device) not the phone.
I was trying to dual audio into a headphone and my Bluetooth paired hearing aids. The "Media output" option (not the "Media" button in my case) showed both, the headphones and the hearing aids, not to select both at the same time, but either one of them at a time.
At someplace I read that the devices must support the option, so I decided to connect another set of headphones (Taotronics TT-BH040) and now the "Media output" option shows both with the selection checkmark at the right and, when both selected, audio is going to both at the same time, and volume can be adjusted independently.
May or may not be your case, I just felt that should share my experience.
