Interesting update on the original post. I recently acquired a second tandem from my local nonprofit bike shop. It's a 1980s era fillet brazed Rodriguez, identical geometry to the Burley duet - - but finally got it rolling after a full overhaul and the difference is incredible! The Rodriguez just feels rock solid. Captain noticed the difference immediately too and felt much more relaxed. The only other difference between the two frames is that the Rodriguez has an oval profile top tube (and, of course, is nicer to look at). Could the method of joining tubes and one oval tube really make *that* big of a difference? Other strange thing, the Rodriguez feels much more squirrelly riding solo than the Burley does.

I don't fully understand what's going on here, but it's encouraging. Taking the rodriguez on a bike camping trip to the San Juans this weekend, so we can get in plenty of miles without too much traffic.