When using an extra monitor I only have 1 desktop on bootcamp Windows side

I have bootcamp on my macbook pro 15" mid, 2012 to run Windows. I bought a mini display port to DVI adapter & am using the correct DVI cords (This is what apple support told me to do). When I plug it in on my Mac side the monitor works as an extra screen (I have desktop 1 & desktop 2 ), but when I use Windows on bootcamp the monitor does not work as an extra screen. It displays what I have on my macbook (so I only have desktop 1). What do I need to do to have desktop 1 & desktop 2 when using bootcamp?