Well....it's not so much alignment of their acoustic centers, as their axes are still separate. What, I believe, you describe is a coaxial driver or a point source. I know what you're saying, however. Perhaps a more complete explanation, is that it's a matter of better time / phase alignment of the sounds (from the separate drivers) reaching your ears at the listening position. This is a large part of what makes designs like Vandersteens and Thiels what they are. The idea is that one can build a simpler crossover, if the drivers can be physically positioned in a time/phase coherent alignment.