Make sure both monitors are connected to the NVIDIA-based graphics card,
Open the NVIDIA Control Panel. You can do this from the popup menu when you rightclick on the desktop.
Under the Display category, select "Set up Multiple Displays"
Check the box next to the inactive display you wish to activate as a secondary display and click apply.
If your display is not shown, click "My display is not shown..." to force detection.
You can drag the green boxes around to match the physical arrangement of your displays, and choose which one is primary by right clicking on it and selecting "Make Primary".