I recently picked up a used Huret Duopar Eco model derailleur. Very interesting design! According to the archives it wasnt exactly the most robust derailleur ever made. Rolling a bike backwards could cause it to self destruct! That could definitely be a problem with a tandem. The derailleur did go through a number of small refinements over the years. This Japanese Huret page shows four Duopars about midway down the page...
http://homepage3.nifty.com/
If you look at the last one it is a late Sachs/Huret labeled version and the arm connecting to the upper pulley has been beefed up quite a bit...
http://homepage3.nifty.com/
Did these late models prove to be any stronger? Thanks for any feedback.
Jim Cole
Memphis, TN