Concentric drivers

I mean if you setup a big speaker on a trampoline and crank up the volume real high, maybe there could be some measurable doppler effect. But there's really not much specifically about a concentric driver to be concerned about.

The woofer does act as a horn extension for a Tannoy's tweeter, but it's never been a topic of discussion or concern, to my knowledge. 

Concentric drivers can’t have the coils aligned, ever. Perhaps the tweeters’ backside structure is acting upon the wavefront from the woofer behind it, but the physics involved goes off into the deep end of the pool imho.... ;)

The alnico/pepperpot Tannoys are supposedly configured so that the tweeter and woofer’s acoustic center are at a distance equal to a half wavelength at their crossover point (1.1 kHz). So guess what, they flip the +/- wires to one of these drivers and voila, phase coherence (at the crossover point) lol. I don’t know if the more modern ferrous/tulip models are configured in this manner.