What about Tekton or McIntosh with their arrays?
There are lots of examples of speakers that do use multiple tweeters properly. Axiom is doing none of that. To address some of the manufacturers named in this thread:
The older Dynaudio confidence speaker low-pass filtered one of the tweeters to a relatively narrow range.
The Tekton speakers with those tweeter grids only allow the central tweeter to operate in a normal tweeter range. As Ryanosaur stated, the rest acted like a midrange driver where the spacing isn't far enough to cause comb-filtering. Also, the surrounding tweeters may be operating as a Bessel array to maintain direcitivity matching and power handling.
The McIntosh speakers, if you are talking about their line arrays, use very close spacing of the tweeters so that they don't act as a point source speaker but rather a line source speaker. You need a specific center-to-center spacing and minimum length array for this to happen. The acoustic theory is a bit complex, and Axiom certainly isn't attempting a line array in their speakers. If you are talking about the
XR100 model, that really only has one tweeter. The rest are midrange drivers that probably aren't comb-filtering since they are relatively close for the frequency bands that they play.
Dali uses two tweeters, but the ribbon is a supertweeter and they don't really have overlapping ranges.