I try regularly to solve this and have and no luck.
With my Samsung qled TV, when using multiview. I can watch you tube and TV and mix the audio. Aka have 50% from TV and 50% from you tube.
When I set it up to TV and xbox series x. This option fully disappears from the multiview settings.
I would love to do the same thing of 50% 50%. So what settings do I need to change to allow this? I'm already aware that the TV cant handle the xbox in 120 hz in multiview and dont mind setting it to 60hz. But the inability to mix the sounds at this point makes multiview very limited in its utility.