My Yamaha DSP has an option for using two centers and I do. It was a big improvement with fuller sound. I use M&K 's which I feel do a great matching job with the Andras I use for mains.
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If you have a timbrel mismatch, adding another speaker will do nothing but give you more sound. That sound is likely to be diffuse. It may sound "fuller" but I'd bet you'll miss the precision on a good sound track.
If voices are not "stuck" to the screen, you may just need a little more volume on the center channel. Make sure your center is as close to the plane of your mains as you can tolerate.
If your sound is hollow in the center, a better center speaker may help but you may also get good results simply by a bit of repositioning of your mains. Don't screw up your 2-channel, though!
Despite all the hype and high prices, it doesn't take much of a center speaker to make HT sound good. Good HT, anyway. You may find, as I and many others have, that the only really satisfactory solution is to have one system for 2-channel and one for HT.
Good luck. Let us know how it comes out.