We are using Di2 2x11 on our triplet. Using the House of Tandems 34T inner and either a 52 or 53 big mounted on the middle chainring position. 11-40 rear cassette with a road link.

Works great. Have a d-fly unit linked up with the Garmin so I can see what gear combination we are in. Gone are the days of asking the rear most rider "What gear are we in?"