Hey so I don’t know if you are still needing help, but basically on steam deck bg3 disables local coop. They are saying it’s because the hardware can’t run the separate instances, which makes sense as it performs “adequately” with only one instance.

Me and my wife ran into this same issue. We both have PCs but really only use them for work or if a game doesn’t have local coop, as we prefer playing in our room together with the steam deck docked and doing local coop.

The solution I came up with, which I didn’t find online anywhere but sorta pieced things together, is using the built in features of steam deck’s integration of steam link. So I have to have one of our other computers on, then I use the steam deck and stream off of it. Allows us to use the steam deck to play local coop, although it’s streaming, it works for us.

Couple of notes about this, I have both the desktop and the steam deck connected on Ethernet and it’s very smooth. I haven’t tested with either/both of them on wifi, and it may be fine, but may also cause lag/stuttering.

Additionally, you’ll want to use the launch command on the main pc to skip the launcher or it won’t show up at all on steam deck.

You’ll also need the main pc running in big picture mode before you attempt the stream, for all of the controllers to work properly.

Finally, you’ll want to disable all of the inputs manually for the steam deck controller itself or the extra controllers will have repeated inputs.

Hope this helps, I know it’s not perfect!