Hi willso7,
Using two centers, one above and one below the screen to "phantom-center" the dialog at the screen's center is an excellent idea, and one that we've been recommending at Axiom for some time now.
Whether or not you need to do it depends on the particular installation and the acoustics of your room as well as your seating distance.
If a single center speaker is physically too far away from the screen, it can produce an odd dislocation of the dialog at the speaker rather than at the screen. If the center is 2 feet or less away from the screen, I've found that my brain integrates the dialog with the actors on-screen (I use a 50-inch rear-projection set with an Omnimount shelf to support the VP150 on top of the set).
It's so variable that you have to try different locations for the center speaker in your particular room and see if one works well or not.
Regards,
