Unity isn't great with multi monitor support. The solution is kind of hacky, although it should be easy enough to allow players to position the UI and render the scene on a user specified part of the virtual screen, allowing people to then span the whole thing across monitors. Basically it should be a command line option overriding Unity resolution selection, a ini user specified resolution, a user specified scene render resolution and position, and moveable UI. It's an minority use case, but I wish Obsidian would just dedicate a tiny slice of time to seeing if they could leave in a hidden experimental mode that's not supported or guaranteed to work at all.
Saying that I plan to ditch my multi display setup and go with a 1440p ultrawide or large 4K when I can afford to, after having multi display for 19 years.