The chain pitch (link length) did not change from the standard 1/2". It is only the width that was changed from the 10spd 5.88mm to 5.5mm for the 11spd chain. One of the easiest ways to tell the diffs between chains is to go look at a 3rd party vendor such as KMC's "
chain choice" selector and compare the diffs there.
According to
guesswork from Lennard Zinn and other posts online, people have had success mixing 10 and 11spd components. So, the good news is that it appears we can still use our existing 10spd cranks with the new Shimano 11spd chain and shifters and
it should not be necessary to upgrade 10spd tandem cranks just to get the 11spd cassette gearing. I believe that with the 11spd cassette you would still need the 11spd shifters in order to get the correct amount of cable pull required for that cassette spacing.
One additional irk. At the moment, KMC does not have an 11spd
Missing Link listed as compatible with Shimano chains, and the 11spd versions they do have (Campy) are non-resuable and state those versions need one special tool to install and another tool to remove. Up till now I've enjoyed using the reusable 9 & 10spd Missing Links (very convenient for chain removal when cleaning or travel) without needing any tool other than maybe a pair of needle nose pliers. Reviewing KMC's tools, those appear to be simply narrow nosed pliers, so other than having one for removal, likely only that one is needed (for install, simply pull on the chain). Hopefully there will be more selections available in the future.