Keep in mind that human ears can sense a difference in left vs. right, but have a much more difficult time sensing a difference up or down (within a theater environment anyway), so putting a speaker to the left and another to the right of the screen/tv or whatever will not be as effective in really holding the center channel vocals and sounds to the screen. Your ears will blend them into the right and left front channels. I wouldn't do it, even with a good amount of separation between these two left/right centers and the left/right fronts.
