When you enter Full Screen mode in Parallels Desktop for Mac, Windows (or whichever guest OS you're using) expands to fill one display, but any additional displays connected to your Mac are unusable.
Stop Windows or whichever guest OS you're using.
Update Parallels Desktop to the latest version: In Parallels Desktop choose Parallels Desktop > Check for Updates. Then follow the on-screen instructions.
Once the update is finished, choose Virtual Machine (Actions for Parallels Desktop 10) > Configure > Options Tab.
- Click Full Screen and deselect Use Mac OS X Full Screen.
Now when you enter Full Screen mode in Parallels Desktop, you can continue using additional displays connected to your Mac.