Split-screen support

Multiple controllers can be connected and two players can play together in singleplayer. It is unclear how this would work in multiplayer (multiple accounts signed in at once?)

This is not trivial whatsoever, the client game loop specifically runs logic for the local player and relies on this player instance for a lot of logic, adding a second may complicate logic and mixins a LOT. Maybe make a new LocalPlayer instance that acts as two players at once?

Screens are also something to consider, how do you decide if a Screen should show for one player or the whole game window? How does mod compatibility fit into this?

Start by tracing the client game loop and how the local player instance is used. Then investigate LocalPlayer, Screens, multiplayer account handling, and mod compatibility to define whether two controllers can share one instance or require separate players. Done means split-screen behavior and screen ownership are specified for singleplayer and multiplayer without breaking existing mods.