The two M's of an MTM act almost as one unit, sort of like one big odd-shaped cone (with a gap where the tweeter is) in the vertical plane. Just as a ribbon's pattern is wide in the horizontal plane and narrow in the vertical, so too with the woofers in an MTM. Now the lobing pattern of an MTM is different from what it would be if that were just one big oval woofer, so it's not an exact correspondence, but it's pretty close.Some manufacturers prefer to go with a TMM layout, as this way there's less beaming in the vertical plane.
In my opinion Tyler Acoustics is doing the MTM right in their Pro Dynamics series - using a horn tweeter whose pattern is wider than it is tall, roughly corresponding to the pattern of the woofers.