How to Mirror Specific Displays in OS X: Mirror some, but not all of your monitors on an Apple system

Selective Display Mirroring in Mac OS X: Mirroring tips for 3 & 4 display setups

Many, if not most, professional Mac users prefer a multiple monitor setup. Sometimes these setups extend to 3 or 4 monitors. In this scenario it can be handy to have two separate displays, each with a mirror in another room. Or perhaps an audio professional may use 3 separate displays in a recording studio, with a mirror of your main monitor for external viewing/operation in the recording booth.
This all sounds great, but achieving this kind of setup can be a little tricky in OS X, Apple don’t exactly make it clear that this option is even available.
Don’t worry though, this option is available and below is a step by step guide on how to do it.

How to get to Display Options in Mac OS X

OS X System Preferences Displays to mirror specific displays
To get started, just head to ‘System Preferences’ and click the ‘Displays’ option. This will take you to the ‘Displays’ preferences screen where we then can mirror specific displays. You should now be presented with a screen like the one below:
4 Displays in OS X

How to Set Up 4 Displays with 1 Mirrored Monitor

Main display mirrored and 3 seperate monitors OS X
To achieve a monitor set up like the one above, may seem impossible at first, but it’s actually really simple, Apple just don’t tell you how to do it. All you need to do is hold the ‘Alt’ key on your keyboard and then drag one of your secondary monitors onto your main monitor (your main monitor is identifiable by the menu bar on the top of the screen). By using the ‘Alt’ key, we can now mirror specific monitors. In the set up above we have 3 separate independent displays and one monitor that mirrors the main monitor.
You can also mirror a secondary monitor, this set up would look like the one below. Simply drag your desired mirror onto a secondary display rather than your primary one.
3 Seperate displays with secondary display mirrored

How to Set Up 4 Displays with 2 Mirrored Monitors

2 seperate displays that are both mirrored OS X
To achieve a set up like the one above, 4 displays, 2 sets of mirrors, you just have to use the same ‘Alt’ dragging technique. Please note, in testing it seems you must first mirror the main display and then the secondaries. For some strange reason, it just reverts back to one mirror if you do it the other way around.

There you have it, specific monitor mirroring in OS X explained. If you have any questions just let us know in the comments section below.