I think there is a lot going on with Danny Ritchie's design.
Most proponents of this set-up keep the shared air mass between the woofers to a minimum. There is a limit because you don't want the "port" air velocity too high otherwise chuffing (and distortion) will occur.
The offset design probably introduced additional reflections, turbulance and high air mass. Maybe this is why the results weren't so good.
I suggest you track down others who have made (and still use) these systems to get their feedback.
I've been trying to track down those who use such a system
Hence my post here (and with the clever folk at HTGuide).
Alas, the answers have diverged.
I'm generally not a fan of Mr Ritchie's designs, and this one did not work as he claimed it might.
To his credit he admitted this.
The closest I could find elsewhere were the various SLOB discussions at DIYAudio and the Cardiod Bass ones which led to the Dutch and Dutch speakers.
You, however, seem to know folk who have tried such a system.
If you can direct me, I'm all ears (and fingers).
Otherwise I suspect I'll have to take the non-lazy route and do a test baffle and measure the damn thing.