On the way home from a frustrating day in traffic, I thought of a new way to put together a tall bike:
I welded a seatpost to the downtube of one mountain bike, and just dropped it into the seat tube of another, right where the old seat tube (the one that used to hold my butt up) used to be.
Then I drilled through the steering tube on the bottom bike, and mounted a quick-release axle.
Now the top frame is connected to the lower frame in two places:
The seat tube welded to the top frame, and the quick release axle mounted to the lower frames handle-bar headset.
I can pop the quick release skewer on the top fork, loosen the quick-release seat clamp on the lower frame, pull the chain off the top frame's chainring, and viola...
the bikes come apart!!
I'll post some pics tomorrow, when I can get some daylight shots.

Little nervous about the stress of the welding on that quick release axle... Oh well, it's only blood.