You must be talking about a DBK10? The DB212 is isobaric already (2 drivers coupled by air cavity) - but you knew this right?
Maybe to your interest, I have built a DB210 (2 x 10") and am very happy with the results.
The drivers were not the ones Steve suggests; mine were more suited to a reflex enclosure, but coupling them has given me a result in a box modelled off the DB212 size and shape that I am happy with.

A few advantages of Isobaric is lower mechanical distortion, and as you said, halving volume required for enclosure, and doubling the power handling for your speaker/sub.
A disadvantage is the efficiency of isobaric config drops 3db compared to the single driver.