I think it could be possible to make it work like that but I'm not familiar with Steamdeck itself so I can't say for sure. Need to look into it. Just so I understood you correctly, you want to host the session on the steamdeck and control your own character via the deck, while at the same time your friend uses their own smartphone to control their character while looking at your steamdeck?
Ah ok, now I understand. I don't have a steam deck at hand but based on a quick google search I think it should be possible. The key is whether you are able to connect the steamdecks to the PC so that the Javascript Gamepad API recognizes the decks as controllers. You can connect the decks to the PC and if they show up on this site: https://hardwaretester.com/gamepad they should work with Gaming Couch!
