If “Use second display” is disabled in the Preview window, a message at the bottom of the window indicates what needs to change to enable dual-screen use. The requirements are:

  1. No other app can be fullscreen on the second display.
  2. The second display needs to run at a resolution smaller or equal to that of the main display.
  3. “Mirror Displays” (unrelated to use of a mirror enclosure) in the Displays System Settings must be turned off.

In most cases, a button is provided to make the necessary changes, similar to the following:

Quickfix button to enable use second display

The “Change Resolution” button chooses a compatible resolution. You may choose a specific compatible resolution with the pulldown menu on the button:

Change resolution button showing menu items

If a “System Settings” button appears, it opens the Displays System Settings. In rare situations, it may be necessary to increase the resolution of your main screen so the secondary screen can run at a smaller or equal resolution.