Are you saying a projection screen? If you mounted it above the screen, then it makes sense that the sound would seem to be coming from above, that is where you mounted it? Anyway, you normally want to mount the center at the same height as the tweeters of your left/right mains, which should all be close to ear level when seating. This will give you the best soundstage.
Now some people do run two centers, but a lot depends on what your using to drive them? Also, depending on the "ohm" rating of your centers will determine if your receiver can drive two of them on the same channel. I believe using two 8ohm rated centers would be like a single 4ohm center(not sure how to do the math).
