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?

The Essential Mod supports multiple accounts to be switched within the main menu. Maybe add support for that and use the added accounts for the other players in multiplayer. Also maybe add support for multiple monitors, so player 1 can be on monitor A and player 2 and be on Monitor B.

Start offline with MultiMC

Allow instances to save the prefered controller xinput1, xinput2 etc

I describe this in more detail in #66 .

Start offline with MultiMC

Allow instances to save the prefered controller xinput1, xinput2 etc