JaccoW Challenging project! Wishing you the best.
Tandems are really unique in so many ways that the only commonality with two wheelers is, well two wheels!
Like
noglider , I have a Burley, Mine is the Duet from about 1997. Lots of differences from yours.
WRT spreading the rear, be sure to support the "bridges" on the stays with clamps, you don't want to break those joints!
BTW: if you want to include someone in your text like i did above, use the @ in front of their login name, no space.
Before we stared using the tandem, I looked up how to ride one. There are rules. First is that the stokers is always right. There are others. I added one for my wife, don't rock or lean to the side, especially when turning! Reviewing the rules of what to do to get going and to stopping often addresses some of the fear factor. It is a lot easier for the stoker on a tandem than riding single. all you need to do is hang on and pedal. It also requires a degree of trust in the captain, a good thing to reinforce!