Limited support: some broken menus require multiple resolution changes during play.
Note that in resolutions up to 3840x720, support is native: all menus are functional.
1. FIRST RUN: REGEDIT
By default the game uses the maximum resolution available on your system.
However the main menu is broken in most multi-mon res's, so make sure the game starts in single-screen:
HKCU\Software\Nival Red\Prime World: Defenders Demo
ResolutionHeight_h1843629172
ResolutionWidth_h1003235053
Leave the h* numbers alone, just input your desired height/width and start the game.
2. DURING PLAY: CHANGE RES IN A MISSION
From the main menu (single-screen), click Play then select a mission on the world map.
Then bring the options menu and switch to multi-mon.
Play !
Before you exit the game, switch back to single-screen from inside any mission (since you can't go back to the main menu, and it's broken anyway).