Perhaps I, for one, don't understand how Di2 works. My understanding was that one pressed the up or down button and the electronics put the bike into the next higher or lower gear, meaning higher or lower in gear-inches. Thus one wouldn't care what was being shifted nor would one have a choice of what shifted. Thus in my Di2 chart in post 2, changing gears in the midrange would involve one shift in both front and back every time one pressed a button.
Is this correct? If so, to you Di2 users, does that work on a tandem? And what do you do when the next change is only 2 or 3 gear inches?
I was quite surprised to see that the Di2 spacing throughout the same gear-inch range was almost identical to what I'm getting with our 9-speed triple.