[Question] Does the steamdeck/emudeck support wireless multiplayer?

You can create ad-hoc wireless networks in desktop mode, which should enable you to achieve that. Then, if your emulator supports netplay, with the two SDs connected to each other, you should be able to play by connecting to localhost

That's in theory, in practice, we tried with a friend once, and his SD would just not connect to the ad-hoc network I had setup on mine