depending on your graphic card.. i dunno. but, yeah i think this happens if the video mirroring property thing of your video card is enabled...
try this go to display properties > advanced > u'r card and check the settings and stuff.. disable the mirroring function
edit: actually after thinking about it.. there should be many settings to choose from. i'm not sure anymore if disabling mirroring will help 
.. hopefully it will.
Edit: btw to use u'r second monitor (while playing a game FULL-SCREEN on the other) you have to switch away from the game (minimize it etc.) otherwise i doubt you will be able to enter and work with other applications on the other screen.
but i'm only assuming that.
/ 
gif 
