The idea was to fly it on standard 18mm motors using gap staging.  Since the motors are not taped together, a staging coupler is used to join the rocket.  The sustainer tapers to BT-50 at its boattail where the coupler slides in.  Concentric to that is the BT-20 motor tube.

And it all works fine......BUT

Having a tube-within-a-tube-within-a-tube configuration results in added weight in the aft end of the sustainer necessitating noseweight for stability (of the sustainer; it's quite stable with the booster attached).

As a result of all this weight, it tends to lumber off the rod on standard 18mm motors (eg, B6 and C6 boosters).  If I had to do it all over again, I would attempt to build "light" so that it can get off the rod with a little more authority.  Of course, that's easier said than done.   (If somebody made high thrust booster motors such as the B14 or C5, that would be nice, too )
 

So, if I do another BT-55 Midget, I have a couple things to try.  But I am quite proud of this rocket.  It was pictured in Sport Rocketry in Nov-Dec 2002.  And that's my mug next to it

In the photo below, the coupler fits into the annular space between the motor tube and the boattail. There is just enough space for a motor hook, too.  The gap staging vent hole is just visible below the coupler.