if you have this setting under System Preferences > Mission Control enabled,
then you wont hit this problem. But if you're like me and have it disabled, you might be wondering why your secondary monitor stays black. keep on reading...
The deal with the setting I mentioned in the beginning has to do with what happen when you attach a second, or third, or 'n'th monitor to your Mac. If you have the setting enabled, it will create a Space for each monitor. But if you disable it, all monitors will share the same space.
This isn't really an issue until you want your app to be fullscreened. When a window is changed to full screen, MacOS will create a new exclusive space for it and move the window there.
So when you make 1 of your window a fullscreen window, a new exclusive space will be created, and only your window will be placed there. To add your other windows to this fullscreen space, you need to do this to your window: