The midwoofer-tweeter-midwoofer loudspeaker configuration (called MTM, for short) was a design arrangement from the late 1960s that suffered from serious lobing issues that prevented its popularity until it was perfected by Joseph D'Appolito as a way of correcting the inherent lobe tilting of a typical mid-tweeter (MT) configuration, at the crossover frequency, unless time-aligned.
There are speakers where the tweeter appears below the larger driver and though electrically it is identical to the MT configuration, it is customarily denoted as "TM".
Thus, with a typical TM or MT loudspeaker where the drivers are not time-aligned, the main lobe is tilted away from the horizontal.
There is however a price to pay for this - although the MTM's on-axis response is near perfect, its radiation pattern or main lobe is much narrower than that of the MT or TM configuration's.
As a result, the off-axis response (i.e., response at locations vertically away from exactly opposite the tweeter) is much weaker and there can be obvious and discernible change in tonality at the crossover as one's listening height relative to the tweeter changes, and the effect is perceived more as the listener moves closer to the loudspeaker.
Since the basic reason for this is the distance between the midwoofers, again the solution is to use as small drivers as possible, placed as close as possible.