Hi, I suppose you've installed Nvidia drivers. Then, in your desktop, open a terminal or press Ctrl+F2 and type:
This opens the nvidia-driver window, where you can find the appropiate option.
If you have not installed nvidia driver, install it (via RPMFusion, or downloading directly from Nvidia web).
Also, you can do this editing /etc/X11/xorg.conf file (in Fedora, in others can be in other location), but I haven't try it yet (I use the first method).
Finally, while trying to configure this (using nvidia driver method), have the second screen attached and on, of course.
Please, give more details if any problem.