Originally Posted by
waynesulak
True it is the ring count that limits the gears but the primary reason for giving up a triple is often to allow for the right side sync. It seems that the main reasons two rings get used on tandems are either Di2 or right side sync ruling out a triple. I haven 't seen too many posts about a modern double ring, left side sync, mechanical shifting tandem.
IMO, those assumptions are debatable and I've mostly been ignoring mechanical because this thread is about Di2. I will say that after running the Shimano Dura-Ace 9000 mechanical 11spd on my single, the drivetrain is very finicky and no way I would run that on a tandem. Conversely, since Di2 shifting is so precise and headache free (re: auto trims the FD and maintains RD cage alignment without constant tweaking wire tension) that if running Shimano 11spd, Di2 is the only sane choice.
There is only one mfr who promotes RSD/SSD, otherwise it's not that typical of an implementation. I believe Di2 is much more prevalent regardless of what crankset (crossover or SSD) is installed and the primary reasons for using Di2 are entirely unrelated.