TandemGeek

It also didn't show how the dual derailleur concept was technically awkward.

As best as I can recall, when they first set the bike up for the show it had the Di2 front & rear derailleurs on the tandem with the front triple. Shimano's rep noticed and suggested that wasn't cricket since the Di2 front derailleur was not capable of shifting a triple, as could have been inferred / assumed from the show bike set-up. So, the Di2 front derailleur came off and the mechanical triplet FD went on.

But, here's the little problem with that configuration: the "brains" (microprocessor & transmitter/receiver) of the Di2 are housed in the Di2 front derailleur and the Di2 rear derailleur is simply a slave that runs via a wired connection to the Di2 front derailleur. So, for the rear derailleur to work with the tandem in "triple mode" the bike would have to have both the mechanical triple FD and the Di2 derailleur installed somewhere around the rear crank, unless the rear derailleur's signal wire was lengthened to reach a more discrete location for the Di2's front derailleur, perhas in a seat pack on the stoker's saddle.

Again, that's my recollection of how it played out in 2012 and how the system works. After all, there were lots of folks drooling all over the Specialized "concept" tandem a couple years back, a bike that was more bondo & balsa wood than it was a road ready time trial machine once subjected to a critical eye. Just something to keep in mind when looking at "show bikes" and concepts.