Game developers could do it fairly straight forwardly but the demand for multidisplay functionality isnt really there. Supcom had a 2nd monitor option to render a 3d map but i cant think of any other games that do a separate function like that. Ac6 has a minimap? Fat chance getting something that cool and involved with a game you cant completely mod or make yourself though. Itd be really sick to see something like it on ac6 especially. I miss 4/4fa's HUD.
Gaming
Rendering more screens is more demand on the graphics card.
Also, not everyone has multiple screens or their screens laid out in a similar arrangement and/or the same resolutions.
Beyond these though, there shouldn't be that much limitation to the idea.
I currently play Baldurs Gate 3 couch co-op split between two screens. There doesn't seem to be a way to do it natively, since it recognizes each monitor individually, but doesn't see their connections to each other. I have to do a borderless window and then drag one side of it to the other monitor.
Anyway, I wish there were more built-in options to spread a game across multiple screens. Years ago I used to play Rocket League across three monitors so I could see more of the field while playing.
Some games do support this functionality; typically flight or racing sims. Best of luck getting it running.
Yeah racing sims is what brought this question to my mind. My workplace has a racing sim area which has the actuating seat and multiple monitor setup and by god thinking about it in a modern mech game just made me giggle at the thought.