Ok, right click anywhere on your desktop. Select display settings. you should see both monitors. Scroll down and you will an option to extend the displays. Click on that and you should be good to go.
If your using your Nvidia Card scroll down to where it says multiple displays. Open it and set the monitors up.
Once you go back to Xplane go to the graphics area and you will see both monitors. Open up the options and set them as you like. If you need more help please look at the x-plane Manual on-line or look on x-plane.org.