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Please see the official X‑Plane Support page for help.
I am not from Laminar Research; just a flight simmer. I am not a MAC user.
Firstly try and update High Sierra OS to the latest version as there was a problem produced by Apple when it was released after the release of X-Plane 11.
Secondly try and update to the latest version of X-Plane 11. Even if it means using the beta version. If you are running a Steam version you may not be able to update to 11.20 or later as Steam is not the same as the official version of X-plane from Laminar Research.
Thirdly have a look at the following link at http://questions.x-plane.com/16212/imac-add-2-extra-monitors the creator the question has the same issue as yours and is using a higher spec machine. I understand the person who supplied the answer (amelingu) is also a Mac user.
For best effect all screens should be of the same specifications. The only difference being the serial number.
This site is no longer being actively maintained.