2.4GHz Wi-Fi interference between devices in close proximity, even though they're using completely non-overlapping channels?
It depends on the internal architecture of the receivers in each box - these are designed to match a specification and down to a price. 

A radio receiver has two functions - selectivity, and amplification - and a high performance receiver will achieve each function in several stages - alternating between selectivity and gain.

For example, a tuned filter, followed by an RF amplifier, followed by a tuned filter, followed by a mixer (which often has gain), followed by an IF filter, followed by an IF amplifier, and so on.

Note that the main selectivity (channel rejection) is provided by the IF filter, and the front end RF filters will pass the entire 2.4 GHz band, but protect the following amplifier from other strong signals.

A lower cost receiver may omit the first tuned filter ahead of the RF amplifier. It will actually be more sensitive because that filter had some attenuation. However, extremely strong signals - even outside the 2.4GHz band altogether - can overload the receiver, distorting the wanted signals and mixing in components of the unwanted signals. 

That is essentially what is happening here : because the boxes are stacked on top of each other, one's transmission is seen by the next as an extremely strong interfering signal, (in the 2.4GHz band, though outside the desired channel) overloading the receiver.

You may find that Brand B's receiver is more tolerant to overload than Band A's, but really, without getting deeply into the receiver design there's not a lot you can do about in except prevent the overload, by physically separating the boxes.