How close do woofers have to be placed to each other to achieve acoustic coupling?

In 4350, 4355, and 4435, the woofers are about as close to each other as they can be physically, but in, say, Project May, the distance between the woofers centre-to-centre is about 75 cm.

Put differently, at which frequency does acoustic coupling cease to work as a function of the distance between the woofers?