on isobaric dual reflex banpass

I would have to say there is a whole lot more math to it than just cut the box in half.  I also believe that you use the airspace for one full sub. You just don't need the airspace for the extra sub. 

This type of enclosure is probably not recommended by many.  Most people don't like the dual reflex enclosure or isobaric.  You are trying to combine them which I can't possibly see working out the way you want it to.  Unless you know someone who is extremely knowledgeable about building these type of enclosures, I would stay away from it. 

Only benefit to the isobaric enclosure is you can double the power to the enclosure, but you gain no cone area. 

Dual-reflex enclosures or bandpass, you sacrifice so much of the frequency range of the enclosure it will likely be loud, but somewhat of a one note wonder. 

See what other people say, but thats my opinion.  Hope it helps