If any faults are going to show up on any of your bikes- It will be on the Tandem. This is one form of bike that needs good equipment and it has to be set up spot on. One single thing out of kilter can cost a lot of money. Hermes came up with the Obvious fault as inner and outer cables neeed regular checking- and replacing- to keep the deraillers and brifters working.

So start replacing bit by bit till it gets working perfectly. If this means putting the Ultegra Brifters on the Tandem- then do it. Tiagra parts are now 9 speed and not far behind Ultegra for workability and are relatively cheap. So buy a set of Tiagra for the paramount- or even go bar end shifters for it. Tandems do warrant the best you can put on them. That is if you want to keep riding "trouble" free.