Just recently I've tried using two monitors to play Rimworld, but when both are hooked up, clicking out of rimworld freezes it. It's windowed and is not an issue with the fullscreen, as some people have. I've tried looking for answers but haven't seen anything. It's sort of like when I click out it just pauses it, doesn't continue to play even in the background.
Edit: I can still see both of the screen's contents on both sides, so maybe it's just meant to do that. Still, if there's a way I can do things on the other screen while rimworld runs, I'd appreciate help.
I've noted this down as a suggestion. Thanks for reporting! ;)
Turns out this is already implemented, "Run in background" in the options panel!