I'm wondering if an attempt of dynamic split screen was already made in Mugen like this:
Maybye it's more doable at Openbor but it supposes that one camera can show the player 1 and another one the player 2 position.
I'm 100% sure it's doable because utunnels made a demo release about the small screen demo years ago. That can be used for split screen. I think bWWd made a split screen in one of his mods but I doubt it was released. However, I don't know if the He-Man mod has it. I can be wrong about it.
Having a split screen requires farther distance of players in calculation than a particular screen resolution used in the game. For example, video 0 is used, and the hres (horizontal resolution) is 320. Player 1 is at the scrollpos of 0 and Player 2 is farther away from the very end (320) of the screen. I think subject_to_screen 0 is useful since it's all about screens so you can be off screen. There should be a screen boundary for this. You can use drawbox for the boundary. Each camera needs to focus on each player.
EDIT: For Mugen, maybe O_Ilusionista will answer this.