Stacked speakers have long been used in sound reinforcement. For example, Bose makes a speaker with 4 drivers in a vertical array. The effect is to 'fan' the sound in a horizontal plane with less sound going toward the ceiling and floor -- great for church PA systems. An MTM (or WTW) will not be as extreme but you'll get some of this effect with sound distributed in a vertical 'fan' with sound falling off to the right and left. This is another way of saying the same thing others have said. This is not ideal. I use three identical two way monitors even though I have a fourth sitting in a closet (at the time I could only buy them in pairs). ACI (www.audioc.com), for example, mentions that two ways are better for the center channel on their web site and sells their Emerald LEs (350ea) and Saphire III LEs (650ea) in singles (though I've never heard these speakers).
what's so special about center speaker ?
For $300 ~ 500, you can buy a center speaker or a pair of bookshelf. To me a pair of bookshelf seems to be a better choice because you can later use them in a stereo system, or rear surround. Also, I think bookshelf is more traditional/classic speaker design than woofer-tweeter-woofer speaker. Are center speakers specially optimized to work as a center channel besides its shape? Maybe the center speakers are optimized to produce vocal/dialogue?
Ken
Ken