Full Coop (Campaign, arena…)
Play any map or mode in full local co-op! (Or even online using remote play!) Press F9 to add a player, and F10 to remove a player: that’s it.
Source code available here: https://github.com/Denperidge/Overgrowth-Full-Local-Coop
Credit for the art to a great-yet-uncredited friend!
Also many thanks to the Wolfire Discord and other mod developers.