In the interst of using the forum to share knowledge, for those considering Dura Ace Di2 2013, here are some tid bits on the latest news from a fit up on a new tandem:
2013 DA Di2 has really been spiffed up. Setting it up on a tandem is now a piece of cake, as you can buy the wiring (which is now common to all Di2 including Dura-Ace) in different lengths and just plug it together. In prior years you'd had to assemble a custom wire harness. Shimano also now has their own seat post-mounted battery, so Calfee and Zona batteries are now unnecessary.
If you use an 11 speed cassette then you're stuck with what Shimano offers for the casette and no long cage needed. For deep 10 speed casettes, like an 11-36 SRAM, the Yumeya long cage adaptor was made to work with an earlier-gen XTR rear der which happened to be compatible with the early DA Di2, but it is no longer compatible with 2013 DA Di2. So the only option, unfortunately, means buying an entire Ultegra mech rear derailure just to scavange the long cage which will fit on a DA Di2 2013 rear derailuer.
The 2013 DA Di2 does have ANT+ capability--or so I've been told. Don't know a lot about that. What I have heard is that the digital gear display isn't quite ready for prime time; something about the touch screen not being able to distinguish between a finger and rain or sweat. Shimano's not quite there. And I don't know if Garmin has updated their firmware to read the Di2 ANT+ signal.