I wonder if the 6703 middle and inner chainring would work on the 2014 Ultegra crank? It would need a longer spindle, though?
Someone mentioned above that these hi-end triples from Shimano use/d tripleizers, I hadn't realised this until now. I use a Harris/Willow tripleizer on an old Ultegra hollowtech double crank to get the slightly lighter weight of this crank-BB combination and a 172.5mm crank on my Thorn Club Tour. Yes the setup needs a longer BB spindle to make it all fit and ensure the chainrings clear the drive-side chainstay. The limitation is that hollowtech BB spindles come in only a few lengths, compared to good old square taper, so its not possible to as finely tune the chainline and minimse the Q-factor.
I was happy to see these Shimano tripleizers for about $40 on CRC, that's cheaper than the Willow unit I have now, which will not last forever under full touring loads in high mountains, and Shimano rings (especially the smaller ones) have always performed perfectly well for me. Thorn/SJSC also sell a tripleizer which TRH uses on his Raleigh tourer.
I would use a tripleizer on a tandem if necessary, but would probably go with a square taper triple from Thorn, DaVinci or TA, or a flash FSA crank first, because these come with a matching stoker timing-chain crank too.