Interesting response from Brent of Axiom audio reguarding duel tweeders on the VP 150 center.
If duel tweeders cancel them selfs out than duel woofers should do the same. My point here is that duel tweeders are just fine.
This shows a clear misunderstanding of what is going on.
Think of the shape of a wavelength looping up over center then down below center and back up completing a wave. Play the same wave inverted or with the phase reversed and it cancels it out (playing them at the same time). This can happen at any wavelength.
However the size of that cycle varies with frequency. See a wavelength chart here:
http://www.soundoctor.com/freq.htmSo when you have two sources playing the same signal you have potential for cancellation. In the case of two tweeters even delaying one source verses the other by one inch can cause a 15db dip from cancellation while at the same time a one inch movement is only a few degrees of phase rotation on a lower frequency range and will have no effect.
Here is what I mean. This is an actual measurement of the Axiom VP150. The same speaker pictured above. The tough part was getting dead center of the speaker to take the measurement so as to minimize any cancellation. This is the measurement shown by the red line. The green line is a measurement made by moving the mic over 1 inch horizontally. Get the picture?

And as one gets further off axis the response obviously gets worse and worse.