You mean the left side on one monitor and the right side on the other?
From my experience hooking my computer up to my TV, and using it for movies, you can right click on the desktop -> properties ->select the second monitor and tell it to extend the desktop onto the monitor.
Then open the movie, and dont maximise the screen. Just drag the corners out until the movie covers both screens. If you maximise the screen, it'll maximise to one monitor but not both.
