Not too long ago, we sold our old 9 speed tandems and got an 11 speed. Most of the master links for 11 speed chains are designed for one use only, although some people use them more than once. As for strength, we haven’t seen or heard of a problem with broken 11 speed chains on Road tandems, but I’m sure some strong team has broken one somewhere. Finally, I was a bit surprised to see that Co-Motion didn’t recommend removing the drive chain when packing the bike in S&S cases, as I always did with the 9 speeds.