I finally did hear back from Dean Black, although after I had already finished assembling the rocket. According to Black, the aft ends of both the motor tube and upper tube align and there was a parachute used; it was stuffed in the hollow nose cone.
This is my version. Both body tubes are BT80's, 9" each. It has a 24mm motor tube that extends about 0.25" below the aft end of the upper tube. The space between tubes is 1.75" and the tubes are attached using 4 coffee stirrer sticks. The nose cone is a 9" plastic one I got at the CMASS auction last year - prepainted! It weighs in at 3.5 oz.
I'll give it a try at the April Amesbury launch. It will definitely be a 'heads up' launch!