Only a single input was supported (it could be the built in steam deck controls or an external bluetooth dualshock 4 controller) but changing the input modes (disabling steam input, changing the order of the controllers) didn't solve the issue. Until this is solved this game cannot be played split screen on single device using the steam deck!

At the current version the game doesen't allow to create a local (offline) lobby and it has a disclaimer page in the game for that however online or LAN game is possible!