Archived - Mac OS X: How to use multiple displays
If your computer or graphics card has video output ports, you can connect more than one display to your computer.
You need a graphics card and enough video memory installed in your computer as well as a video cable for each display you want to connect.
Turn off the displays and the computer. For each display you want to use, connect a video cable from the video output port on your computer or graphics card to the video input port on the display. Turn on the displays and computer.
You can set your displays to show the same image (called "mirroring") or an extended desktop. For more information on mirroring, see document 61330: "Mac OS X: How to Set Up Multiple Displays to Show the Same Image (Mirroring)".
Troubleshooting: If you can not see the Arrange tab in Displays preferences, see document 61512: "Mac OS X: Multiple Displays Setup Doesn't Work".
Note: An external display must be connected to a computer to see the setup preferences.
This document contains information from Help Center, the help system included with your computer.