(Help?) 4-Player Splitscreen setup with Steam Input

[Ratirefure]

The goal: Get 4-people, all on Steam Input controllers (i.e. PS4, PS5, Switch Pro, and Steam controllers) playing the game on the same computer (i.e. a docked Steam Deck).

The facts:

• L4D2 has a 2-player splitscreen mode built-in, with various workshop mods to enable this more easily.
  
• L4D2 supports Steam Input (SIAPI), allowing for arbitrary controller bindings, Gyro and Touchpad functionality, and button icons, as well as mouse-like aiming, even for both players in splitscreen.
  
• Steam Input/SIAPI requires the Steam Overlay running
  
• L4D2 can have multiple instances launched directly from the executable with the flag -multirun, but this will launch the game without Steam Input
  
• Nucleus Co-op[nucleus-coop.github.io] exists, but it uses the Goldberg[gitlab.com] Steam emulator, which doesn't have a Steam Overlay (not to mention SIAPI) and thus only supports KB/M and XInput (and it's Windows-only by design).
  
• One can connect two L4D2 games running splitscreen through LAN (sv_lan, sv_allow_lobby_connect_only), and command line options allow for arbitrarily-sized and -placed borderless windows (-w, -h, -x, -y).
  
• All that would be necessary from the above, is to have the game (AND Steam) open twice, each instance covering half of the viewing area, and then activating splitscreen from both instances, then hosting from one, and joining from the other.

What I've tried:
Sandboxing with either Flatpak Steam[flathub.org] or Conty[github.com] on Steam Deck. These work fine, but controllers are shared between them and input doubled. The downside here is that Steam Input provides no way of isolating controllers, or disabling them per-game, and specifically SIAPI games don't let you change the order of controllers (this is just wonderful on Deck btw, which has a permanently-attached controller that always wants to be Player 1).

I'm wondering if there's a way of either isolating (wireless) controllers to specific Steam instances or L4D2 instances, or just a way of having more screens ingame (unlikely). I've been talking about Steam Deck, but I wouldn't mind a Windows-related lead either.

[R󠀡F]

I dont think you can even do 2 player splitsceen properly in this game with Steam Input. Try the built in source engine xinput support instead.

[Ratirefure]

Originally posted by R󠀡F:

I dont think you can even do 2 player splitsceen properly in this game with Steam Input. Try the built in source engine xinput support instead.

I made this thread specifically because I'd rather not do that. The game's splitscreen does work with Steam Input, the only real issue being that the game seems to just select the first 2 controllers it finds with no ability to switch them. I can do this with a PS5 controller and a Steam controller, both using Gyro/trackpad stuff, just fine, even with the right icons, the only issue I'm having is isolating controllers to specific instances of the game.