Larian Studios Forums Divinity: Original Sin 2 Divinity - Original Sin 2 - General Local Co-op in Divinity Original Sin 2?

By default, outside of combat the screen will only split when the characters move away from each other. There is an option to always use split screen outside of combat, even when the characters are together, but no way to prevent the screen from splitting or force characters to stay close enough that it doesn't split.
In combat, if both characters are involved there will just be one screen (you can manually force split screen if the non-active player opens the in-game menu).