Back to Back

This design was thought up by someone who wanted to reap the advantages of cancelling driver non-linearities without having to resort to the "clamshell" lloading and it's inherent cosmetic problem (namely that of hiding an exposed subwoofer basket). This design, like its cousin the "piggy-back" also has several detractions that make it an undesireable choice:

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It shares the same problems with the added springy mass of air that couples the two drivers but with the back-to-back isoload, this problem is made even worse by the fact that the coupling chamber is now even larger, adding more moving mass and springiness over the tunnel-load and thus making frequency response predictions even more difficult.
The increased coupling chamber (pink volume) means that the blue volume and thus the entire enclosure must be even larger, even more closely approaching the volume of a conventionally loaded single subwoofer. In a home this might not be a problem, but in the vehicle where space is at a premium, this is a definite disadvantage!
Now that both magnet structures are in identical cooling environments, they will more closely track each other's performance but unfortunately, now we have two heat dissipating structures in the same tiny enclosure which will greatly reduce the thermal power handling of both drivers, not to mention the fact that as the air heats up, it expands thus pushing each of the subs outward and thus further limiting output by reducing each driver's potential excursion!

While the original creator of this design should be given a pat on the back for creativity, it is definitely not an alignment that we recommend under any circumstances.