how to make dual monitors work as separate monitors and not duplicate? Linux mint 14

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http://unix.stackexchange.com – Need bit of help on this. I am using Linux mint 14 with xfce. I have 2 monitors. LG and Samsung. One is attached to digital input and one to VGA input cable. NVidia graphics card. The 2 monitors come up OK, and I see the Linux desktop on both of them just fine. The problem is that they are duplicate of each others. Kinda' useless. anything I do on one monitor appears exactly on the second monitor. So I like to change this so that I can have one as primary monitor, and the second one to use by dragging windows to it when I run out of space on my primary monitor. I set this very easily on (HowTos)