I just bought the game as of 5 min ago! The first thing I've noticed is it defaults to one of my three monitors and it doesn't seem to be able to scale across them either. Will multi-monitor be supported in the main release? Would it be possible to add a "monitor select" section to the graphics settings as well?
PA don't use traditional modal fullscreen, it's use borderless one. E.g it's same window just because borders. But devs only add fullscreen option yesterday, so it's might not work properly. I sure it's will work much better in future. Currently you can try script from my signature, it's also might let you play on all 3 monitors like in fullscreen.
PA will be multi monitor. They have some really cool plans for multi monitor functionality, it's just not there yet.