Eh hem!...Subwoofers... What do ya know?

davekayc, two drivers operating in phase with each other are acting acoustically as one driver if they are within 1/2 wave length of each other at their highest operational frequency. It does not matter where they are pointed. As an example if the woofers are crossed out at 100 Hz, 100 Hz has a wavelength of 10 feet. If the woofers are closer than 5 feet they are functionally one driver assuming they are operating in phase with each other. I have two 12" drivers pointed right at each other 3 feet apart right up against the middle of the front wall. There are two other identical woofers one in each front corner. They are placed so that the edge of the driver is right up against a wall. They are over twice as efficient this way. They are all acting as one driver, one wave front. Since the woofers are right up against the wall the first reflection off the front wall is exactly the same as the initial propagation so essentially there is no first or early reflection. Since the drivers form an infinite line source there is no reflection from the side walls either. 

I would think that within a certain distance . One opposing driver would either assist or hinder the other at a given frequency. Taking all walls and reflective surfaces out of the equation . Dead of space. Think about a truly tuned exhaust. It reflects a sound wave within to increase power . At some point i would assume the area of an 18” speaker would be affected by the other . In phase. Question is how. 

http://www.lansingheritage.org/html/jbl/reference/technical/1983-subs.htm
This is the design used to build the subs . I built them exactly to design , 12cuft version