MTM vs TM ... advantages/disadvantages ?

Two mids driving the same frequencies will have an interference pattern. The response will be greatest when the ear is at the horizontal level of the tweeter (mid point). The off axis response (ear position above or below the mid point) will have some cancellations at certain frequenices. In practice this makes the "MTM" a poor design over TM for near field applications where your seated or standing position may vary. For applications at a distance this design works well (as in vertical stacked arrays at a sports stadium), the sound beams out from a line array in more of a cylindrical pattern than spherical, and for lagre arrays the sound decays at 3 db per distance doubled rather than 6 db (useful when you want good audibility at a far distance in the rear without making the people at the front suffer from overly loud sound)

Very interesting ! From Shadorne's perspective , the MTM would not be a wise choice in a nearfield situation such as mine .
Very helpful and kept me from making a mistake !

As always , a wealth of information at my fingertips !
Thanks people .

My MTM Thor speaker delivers a much richer/fuller/bloosed sound stage/image, vs my old speaker with a single 7 inch midbass/midrange.

Shadorne...The off axis "lobing" which you cite is, in my experience, not a real world problem. Perhaps this is because the drivers flanking the tweeter are usually as close together as possible, (1 foot for the 7 inch drivers of my Madisound Odins) and have considerable surface area so that it is hard to say how far apart they are (acoustically). Furthermore, this lobing would occur for off axis listening in the vertical direction, where the off axis angle will not be large, and is easily minimized by tipping the speaker system. The effect is, however, why you should not generally use MTM speakers mounted horizontally.

Oh, and by the way, two driver speakers also exhibit lobing in the crossover frequency range where both drivers are active.