Boxee with a twin display arrangement extends over 2 displays in fullscreen
You can push
\ to toggle complete screen setting. After that you can at the very least run it in windowed setting on a solitary screen.
For an extra full remedy, this thread on the Boxee forum might be handy. It is been a while given that I made use of Boxee on a twin screen system, yet when I did, I made use of something comparable to what is synopsis there.