It's a good article, but the 2H cancellation he gets is by operating them into a common chamber and keeping them very tightly spaced, the latter of which I mentioned above. EV did a direct radiator push pull PA sub some years ago, with the drivers going into a common manifold, very similar to what WP did with the Pi12, but without the horn attached to the front. M&K could have done the same, but it does make it difficult to build in some form factors.
The attachments make it easier to visualise in the drawing (plenum can be slanted or square to baffle) and the second is a finished build by Les Hudson who posted about it at AA and is based upon the work of EV and poster 'djk'. It's a 2x12" push pull slot loaded ported.