My HT has two Axiom VP-150s, one beneath the screen on a shelf and one above screen on a mounting bracket (92" screen). I did it for similar reasons saddlesore – I started with one center below the screen and had issues getting it to sound right in many circumstances. For the long-story, even before that I started with a perforated screen and a center behind it….great sound but not-so-great picture. And I’m sitting fairly close to the screen which gives less room for error. Two centers has fixed everything (pretty much, keep reading) - my virtual center is now "centered" horizontally and vertically and seems fuller in general.

My solution to get two center channels out of a 7-channel amp is to skip 7.1 and go 6.1. That way you have an extra channel for two centers. The only other way to do it in my opinion is to get another 2-channel amp and let one of the channels in your 7-channel sit idle (in that article you linked they actually used 4 channels of the receiver for surrounds and then two 2-channel amps for R,L, CT, CB for a total of 8 channels). I considered getting a 2-channel amp for the surrounds to get to 7.1...and probably will some day, but so far the single rear speaker has been working okay for me.

As for cancellation…I did need to do some tinkering on placement to get them to sound like a single speaker. My setup helps me do this in that the three primary listen positions are all in a row the same distance from the screen. So, since the top center is little farther from the ear in the vertical space, I have it little closer in the horizontal space so that the ear-to-speaker distance is the same. I tweaked the angles a bit two. I also carefully set the levels so that each center was equal but in sum equaled the other speakers. This is not much extra work than careful speaker placement and setup in general though. And I would recommend the exact same make/model of center-channel speakers. To be fair, I do have one non-primary listening position where it doesn’t sound quite right that may be the fault of the two centers canceling or something…may need to work on that one a bit or give it up.

All in all, I’d recommend two centers, more amps if desired to get to 7.1, and more patience on setup if needed – it is worth it. You can’t skimp on center on HT in my opinion, and so if you aren’t satisfied and are willing to spend some time and a few $ for another center, go for it. To save $ (like me) and not buy more amps, I’d say two centers (in my case and maybe yours) trumps 7.1 over 6.1.