Ctrl + Left and
Ctrl + Right Keyboard shortcuts only work on the monitor where the mouse pointer is located.
Command + Tab If you're using an app on another monitor and trying to switch places, it will not work on the app I switched to.
Also, try switching the blanks on one screen, and then move the pointer to your other monitor without clicking or selecting anything. If you now use the keyboard shortcuts to switch, the blanks on the other monitor will be changed, even if the focus does not exist.
Is there a way to change spaces only on the monitor that has the focus? Incidentally, I'm on the latest Mojave.