stokid python, and should solve one mayor problem: with Vincent in the front we'll be able to watch over him.
The wheelbase will be 1650mm, with 26" rear and 20" front wheel. As a basis I will be using a CrMo mountain bike frame I have lying in my shed. The plan is to cut the frame through in the middle and put the middle part of the tandem in between. As simple as that...
I have a second design I am considering. The stokers handlebar will be attached directly to the fork-stem of the front fork. The advantage would be a simple construction. On the other hand, I'll have to cut handlebar narrow so the stoker won't be able to steer. A drawing of this setup:
I think I like the second design better, may be simply because of the novelty of having a tandem steer.