Unfortunately that is not possible and probably won't be in the future. The reason is that multiplayer is built around having one "host" player who is the main player on the farm. When someone joins via split-screen, it's relying on the first player being the "host". I don't know for certain as I'm not a developer, but I suspect that a feature allowing two non-host players to play in split-screen would require some significant changes to the game.
I'd be happy to be proven wrong on this though, since I do think that would be a great feature for the game to have.
With two instances of the game running on one machine it is possible in some way. Just start the game twice.
The first instance would be the host, the other just a client. The client has to connect to 127.0.0.1 for the "host address" (it's the self-address of the machine).
This way you get two game-windows in one coop-game which can be placed each as a half desktop...
For 4 players one has to start the game on a second machine twice as well, but connect to the intranet-address of the first machine (with the host).
The thing is, one has to rebind the keys still for only one of the game instances (on each machine used). So each player has keys for its own!
This is probable possible, but I haven't tried it. Maybe someone else has a nice idea or even a tool to do so...!?
If this is possible, help!
Edit: I forgot that one can rebind the keys in the game itself. Just scroll fully down in the options menu. Then select none-used keys for the second instance (the second player).
Ready!
So yes, it is possible.
