That's an interesting concept for a very specific use-case - one in which the user requires the output of a dual 18" subwoofer, but cannot accommodate either the added width, added height or added exposed surface area (front + side, side + side, bottom + side, etc.) of a traditional dual driver design. You also seem to be going for more output than something like the MK Sound subwoofer design where the second driver fires into the cabinet in order to increase the X-max of the primary, outward-facing driver.
My biggest worry would be tuning the correct summation of the group delay. You are already dealing with the group delay of the traditional port + driver design at the front. And now you are adding the delay of the horn output. Just seems like it would take A LOT of careful design work. You're also dealing with the dual resonance of the cabinet, since you have the traditional port resonance, but are now adding a folded horn to that same cabinet. I suppose you could massively damp the horn with a constrained layer design, but then you would be losing a ton of the reverberant output of the horn design (which would seem to be the point of using a horn in the first place!).
What about a hybrid sealed + horn design? As in, the front-facing driver is sealed, rather than ported, but you still use the folded horn for the second driver. That could eliminate some of the group delay and dual resonance issues. You wouldn't get the same, massive output, but I would venture that the folded horn alone would still provide massive output while the addition of the sealed front-facing driver would boost output even further, but avoid so many complications with the delay and dual resonance.
I'm not sure how often this specific use-case would come up, but, if for some reason, the exposed surface area of the sub must be kept to a minimum, I could see this being an effective way of drastically increasing the output of what would otherwise have to be a single, front-facing 18" driver design!
