HOW TO PLAY 2 PLAYERS (1P KEYBOARD VS. 2P CONTROLLER)

1. Go to your Steam Library, right-click the game → Properties → Controller → Disable Steam Input.
   (If possible, also disable any Steam Input settings related to the controller. From what I found with GPT, enabling Steam Input may cause the game to automatically detect the controller as Player 1.)
2. Launch the game and enter VS mode.
3. Once in VS mode, press Shift + Tab and open the Controller Settings (the controller icon).
4. There is an option to toggle “Disable Steam Input / Enable Steam Input.”
   Keep switching it and go back into the game to test each time.

After toggling it a few times between enabling and disabling Steam Input, the game suddenly started separating the keyboard and controller correctly.

(In my case, the controller was sometimes controlling both P1 and P2 at the same time. After going back into Shift + Tab and toggling Steam Input again, it eventually worked and separated them properly.)

I spent a really long time on this and only got it working by accident, so I’m sharing it here in case it helps someone else!