Get Complex Multiple-Display Setups

If you have three or more displays or projectors attached to your Mac, you might want to selectively mirror the content of one display on another yet use the others as independent monitors with their own desktops. Although there’s a Mirrors Displays checkbox in the Displays pane of System Preferences when the Arrangement tab is selected, it’s not designed for a setup this complicated.

Setting Up Mirroring 

The solution is surprisingly simple. Open System Preferences (Apple menu→ System Preferences), switch to the Displays pane, and select the Arrangement tab. Then turn off display mirroring completely, as described earlier, so that each screen has its own desktop. Then hold down Option and drag and drop the first of the screens onto the screen representing the display or projector you’d like to mirror it (note how the border of the screen or projector you want to mirror the display upon will have a red border, making it easy to see what’s what).
You should then find that those two displays are mirrored. This can be repeated for any other displays you’d like to mirror.

Deactivating Mirroring 

To return each display to its own discrete desktop, again hold down Option and drag the displays so they no longer appear to be on top of each other and are lined up separately side-by-side.
Incidentally, to make any particular monitor or projector the default, which is to say the one that shows the Dock and menu bar, click and drag the white strip shown at the top of one of the blue screen previews within the Arrange- ment tab to whichever device you like. This will then become the default display whenever it’s attached.