Here's a very special Tandem for sale, custom made by the famous Merlin Metalworks company. They made very, very few of these, and from what the original owner told me, it could even be the only one other than a few 90s mountain tandems. This example was built in the mid 2010s, and the frame alone cost around $10 000 to build. It's equipped with a 2x10s Ultegra Di2 drive train with a pair of bullet proof, custom built, DT Swiss TK540 wheels and Carbon FSA cranks.

Initially built for a ~5'5 stroker and a ~5'10 steerer.