@mijostyn, allow me if I may to let you in on what Magnepan is up to with this new design. Planar enthusiasts have long searched for a woofer system to compensate for one of that design's major failings---output below, say, 100Hz, due to, amongst other reasons, the side-null dipole cancellation inherent in planars.
SoundLabs addresses the issue by making ESL speakers with very large radiating surfaces. Magnepan did the same with the original Tympani's in the 1970's, and most recently with the 30.7. But all of those require very large rooms---the bass panels of each channel of my Tympani T-IVa's are 36" wide!
Dynamic cone woofers have been tried in every way imaginable, including the way you suggest above, and all have been found to be unacceptable. With one exception: dynamic cone woofers implemented in not in sealed or ported enclosures, and not on an infinite baffle, but installed in an Open Baffle/Dipole frame---"H" and "W" or "M" being most common.
That is the woofer system Siegfried Linkwitz used in his LX521 loudspeaker, and GR Research/Rythmik use in their servo-feedback version. And it is an OB/Dipole sub that Magnepan is using in this new "dual-dipole" model. OB/Dipole subs are catching on with hardcore planar lovers, including owners of Maggies, Quads, Acoustats, Eminent Technology's, and even high-transparency non-planars. Using dynamic cones implemented as OB/Dipole radiators with planars cures the inability to get the two to blend seamlessly. Why that is is beyond the scope of this post.