Not sure what the actual name of it is, but this is part of a correspondence between Scott Lindgren (scottmoose on diyaudio.com) and I on the subject.
Yes, adding a second driver mounted through the internal baffle is an interesting idea for a bit of tapped-horn boost. Note that Vb will have to double though -something of a downside when a cabinet is already rather large. ;^) Wish I'd thought of it, but GM takes credit for that one. No, it wouldn't necessarily have to be at the same height as the forward-firing unit, though it it would be advantageous IMO. Overall, I suspect it would end up being somewhat design-specific as to where it will best be positioned. No, you've worded your question correctly, I can't see there being a major problem of cancellation occuring, assuming the correct placement of the drivers so that the wavefronts do not encounter something in the reverse-phase.
