I have a dual screen setup (4K and 1920x1200 screens arranged horizontally), and when I go into fullscreen mode, the game zooms to expand on both screens at the same time. It also keeps the proper aspect, so only the top half of the game is visible (can't click on the checkbox anymore because it is now offscreen)
Also, I noticed that when the game starts initially, the window is not centered but somewhere on my bottom right corner. It looks like it sees a combined screen of 3840 + 1920 = 5760px, and if the aspect ratio is preserved, probably 2880px high. I guess if it would center itself on this huge screen, it would end up on the bottom right corner of my primary screen (4K display).
I'm on win10 creators update, and game is InDev v0.38.