Depends on your graphics card. If you use nVidia, then firstly you should be using the restricted drivers. If you are,
1) then press Alt+F2 and type in
2)In the window that comes up, type in your password - if required - and then look for TwinView. That's what you need to set. Also mark which one you want to make your primary monitor and which one your secondary.
3)Make sure you click "Save to X config file" or something along those lines. Then either reboot or simply restart X (Ctrl + Alt + Backspace)
If you have an ATI card, then I am not sure.
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