Joseph D'Appolito's original design had a few characteristics which have not made it into everything "MTM".
He used an odd-order crossover (3rd order), which would have a lobe at 45 degrees above the tweeter axis in a normal tweeter-on-top two-way configuration. With the MTM topology, we get one lobe above the tweeter axis and another lobe below the tweeter axis, resulting in significantly more uniform vertical coverage in the crossover region than we'd get with an even-order crossover.
Also, the original tweeter was a horn-loaded Dynaudio dome, and the physical set-back of the tweeter's voice coil due to the horn depth contributed to time-alignment in the crossover region.
Finally, the narrower radiation pattern of the horn-loaded tweeter resulted in better pattern-matching through the crossover region than we'd get with a conventional dome tweeter.
The advantage of the more-commonly-used 4th order crossover for an MTM configuration is that it allows a lower crossover frequency, but along the way some of the desirable attributes of Joseph D'Appolito's exceptionally well-thought-out approach have been traded off.
I've used an MHM format ("H" for horn tweeter) several times over the years, mostly in custom studio monitors.
Duke