You will need to install the NVidia drivers, if you have not already. You should be able to find these under System Settings -> Additional Drivers -> NVIDIA accelerated graphics driver (version current).

Unfortunately, the NVidia binary drivers do not fully stick to standards for describing multiple monitors. The System Settings -> Display window will show only a single large "screen" spanning both monitors. To configure your dual monitors, you need to use the NVidia X Server Settings tool which should have been installed along with the drivers.