On my computer I have to open up the NVidia menu to enable the dvi output. I can then use it alone or with the vga output. I can display the same information on both, have them set as one screen with the top part on either screen and the bottom on the other, or as side-by-side.
I suggest reading your manual and checking out the menus for your video. You may also have to check out the BIOS settings.