Tony, welcome. In the past I did a fair amount of experimenting with center channel configurations(although I didn't have the M2s). In short, I think that what you propose would work well, although I doubt that it's really necessary. The problem that Mark(BigWill)read about somewhere may be "comb-filtering" when two separated speakers reproduce the same material and partially interfere with each other. This does exist, but it can also happen occasionally with the regular two stereo speakers and in my view shouldn't be given much weight, especially considering all the comb-filtering that's happening continuously in any listening room that isn't an anechoic chamber. The center image will shift slightly as the listener moves to the left or right, but won't move out farther than the center speaker on that side, which should be satisfactory.
The reason that I said that it shouldn't be really necessary is that a single vertical speaker which has very wide dispersion should be adequate, as I've found. Despite a rather puzzling recent report of an M22 having a narrow "soundstage", both my personal experience and the NRC measurements show that the Axiom tweeter has very wide dispersion(which is what determines the soundstage when other factors are held equal). So, a single vertical M2 should be very satisfactory, although your experiment will be interesting.
