Troy, although the linked article didn't mention it, the general requirement for two drivers playing simultaneously to "couple" and result in as much as a 6dB level increase(varying somewhat with room acoustics)is that their cone centers be within about 1/4 wavelength of each other. At 80Hz 1/4 wavelength is about 3 1/2'(1130/80/4). The EP800 drivers are already configured that closely, but two EP 500s could be arranged horizontally or vertically closely enough to satisfy the requirement.
Although the extra level increase by being located closely(as much as 6dB rather than the usual 3dB from any two drivers playing simultaneously)can be had, possibly more important overall would be locating separate subs widely apart(e.g., midpoints of opposing side walls or front and back wall or diagonally opposite corners)to partially cancel room modes and lead to smoother bass response.
