Change space of other non-active display via keyboard
There is another question here on Ask Different that answer this question:  Keyboard shorcut to switch focus between multiple displays on OS X 10.9.5 + (mavericks, el capitan, sierra)

Here's the quick summary:

There are some utilities (window managers) that allow you to assign a keyboard shortcut to the focus of a specific monitor.  They are as follows (links provided):


Catchmouse (No longer available so it's a moot point here)
Divvy - $13
SizeUp - $13
Slate - Open Source, Free.  Functional, but there's a lot of manual configuration to be done here and not as easy as the paid software.