You can connect two displays to your graphics card and use both displays at the same time. When a second display is connected, you can either have the same image appear on both displays (known as mirroring) or use the second display to extend the size of the Mac OS desktop (known as extended desktop mode).You need a video cable for each display you want to connect.
To use multiple displays:
1. Turn off the displays and shut down your computer.
2. Connect the video cable from each display to the appropriate port on the graphics card.
3. Turn on the displays and start up your computer.
To adjust the displays, choose System Preferences from the Apple Menu and choose Displays from the View menu. By default, your computer starts up in extended desktop mode. To work in video mirroring mode, turn Video Mirroring on with the Displays icon in the menu bar in Mac OS X. You can press the Command key and the F1 key to switch between video mirroring and extended desktop mode.
Important: For optimal mirroring performance, set both displays to the same resolution and color depth.