Turn off the system, plugg both monitors in, turn system on.
Monitors will both display the same thing till you get to windows. Then your primary monitro will be the only one with a picture. Go to display properties and the settings tab. Chick the advance button and click the tab that shows your video card. In there you will find the setting for enabling dual monitor and you will be given some options on how you would like the system to treat that 2nd monitor.