OS X Yosemite: Connect multiple displays to your Mac
You may be able to connect more than one display to your Mac. To do so, you need a video cable for each display. You might also need an adapter.
After you connect the displays to your Mac, you can choose whether to use them as an extended desktop or for video mirroring.
Connect the displays
For each display, connect a video cable (and adapter, if necessary) from a video output port on your Mac to the video input port on the display. To learn more, see Video ports.
Make sure the cables are securely connected or you may not be able to see an image on the display.
If your Mac has one or more Thunderbolt ports , you can connect up to two displays to those ports. If the displays also have Thunderbolt ports, you can connect one display to another, and then connect one of the displays to a Thunderbolt port on your Mac. Or you can connect each display to different Thunderbolt ports on your Mac.
If a display is not recognized, choose Apple menu > System Preferences on the primary Mac, click Displays, then click Display.
Then click the Detect Displays button.
You can also try putting your Mac to sleep and then waking it.
Set up the displays as an extended desktop
You can arrange your displays in any configuration to create an extended desktop. For example, you can set your displays side by side to create one large continuous desktop.
Choose Apple menu > System Preferences, click Displays, then click Arrangement.
Follow the onscreen instructions.
Set up the displays for video mirroring
Video mirroring shows the entire desktop on each connected display.
Choose Apple menu > System Preferences, click Displays, then click Display.
Set both displays to the same resolution.
Click Arrangement, then select Mirror Displays.
Set up some displays for mirroring and others for an extended desktop
If more than two displays are connected to your Mac, you can specify that some displays use mirroring, showing the entire desktop, and other displays show the extended desktop (displaying the desktop across two or more displays). For example, if you have three displays, you can have two displays showing the same information and the third display showing the extended desktop.
Use the task above to set up all the displays as an extended desktop.
Option-drag a display icon onto another display icon to mirror those two displays.
If you are playing a DVD on a television connected to your Mac, click the Arrangement button in Displays preferences and move the menu bar to the television. If you don’t see an Arrangement button, your Mac is only capable of mirroring.