Why? As long as you drive the stacked pair in-phase, the horisontal dispersion of sound should be identical to the single unit, just twice taller. Of course, the vertical dispersion would be totally changed, but that should not directly affect the soundstage. And since the tweeters are close to each other in each stacked pair, the image should be well focused, at least in theory...

Go get four M22's and experiment... I am curious, too!