Darren back whenever I bought our VP180 I wired it with the VP150 via a parallel connection having the 150 above and the 180 below our 73" screen and the combination didn't work well at all for me. My thought was the same as yours regarding planting the dialog onto the center of the screen but the 180 did such a goodl job on it's own that the 150 simply disappeared in the mix, you can try it but I certainly wouldn't expect any improvement whatsoever.
Having a couple of VP180s as dual centers like JC has would be a hoot I bet.
