I'm guessing no switch pro controller support.
Viewing post in Split screen support is now available in Update 1.05! comments
Copy pasting this cause so many people are asking stuff like this and you all deserve to know:
Running any program via steam will enable steam controller support. You can do this by clicking add a game at the bottom left of your library and then add a non steam game and then selecting the games .exe file (BBPSX.exe)
But wait, there's more! If you have a game you don't play any more that has steam remote play together as an option, you can copy the file name of the executable of that game, IE, castle.exe for castle crashers, then delete every file in the directory of that game, and move the bbpsx files into the game's directory, and rename bbpsx.exe to the name of that game ie castle.exe in this case,then run the game that steam STILL THINKS IS THE DELETED GAME and itll allow you to use steam remote play together to play bloodborne psx with friends. PS. Not affiliated with steam, lilith bloodborne, or fromsoft. just a helpful bunny on da internet. o7