Full screen app on one monitor, Desktop on other

I want to use Terminal in full screen on one monitor while having the desktop accessable on the other. pressing the green button in terminal fills screen 1 with terminal (good), but the other is blank (bad). It would be nice if the other were the desktop or another app.

To sum it up, this is how is is for me now:

Screen 1 | Screen 2
-------------------
Terminal |  Blank

When Terminal is fullscreen, the other monitor becomes blank i.e. nothing on it.

Screen 1 | Screen 2
-------------------
 Blank   | Terminal

The same rule applies regardless of what monitor Terminal is on.

This happens to any and all apps, not just Terminal.

I want it to look like this:

Screen 1 | Screen 2
-------------------
Terminal | Desktop

or:

Screen 1 | Screen 2
-------------------
Desktop  | Terminal

or another example:

Screen 1 | Screen 2
-------------------
 Safari  |  Pages

Ideas?

High Sierra 10.13.6