One problem with having two center speakers directed at the listener on a vertical axis is that the sound coming from them will be exactly the same and that could make for some serious comb-filtering if not done in a highly controlled manner. If you want to hear why this is a problem, take two speakers playing the exact same content, preferably a test tone, and move them around each other at various distances from each other and listen to the changes in the sound.
It's not as difficult as you all make it out to be. Your own posts recognize that it is possible to do. All it takes is a little testing with the position of the centers and the tuning (bass, mids, treble, auto eq calibration setups, etc.)
I have my two centers laid horizontally spaced about 10-12 inches apart (MT TM) and its working out well. It may be that what a lot of people hear as comb/filtering or lobing is just the tuning of their system or the content they are viewing. I've tested this by using a 2 channel output sent to the two centers(when configured as front left / front right) and then switching the same audio signal to the main front left/right (positioned about 7-8 feet from either side of the screen) and have noticed that what I thought might be comb filtering/lobing was actually the content I was viewing. When I did notice any issues, it turned out that the two centers actually handled the sound better than the main front left/right.
You can easily run the same test by using a 2 channel output and switching between speaker A/B within the design outlined in the OP.
The same results apply when I use the two centers as the center channel in 3.0 and up configuration.
And, as I've mentioned in the previous posts, it is a welcomed change/alternative to be able to use the two centers as front Left and Right. It provides a more subtle viewing/listening experience for every day use.
