Probably just a worn ring or cog. They were OOS, that's when one first notices it. Typical massively over-geared tandem. Also they aren't synced up very well. Shots from the front or back when effort is high show a lot of frame flex. The stoker is really reefing on the back of the bike - he rather thrashes around. Looks to me like the stoker compartment is 10%-15% longer than the the usual 28.5". That will cause more frame flex though it can be more aero, depending on the stoker's flexibility. For most recreational teams, I think the standard compartment length is more aero.

My guess is that this fancy bike is a bit lighter than two singles. Plus the usual tandem climb rate is proportional to the average of the 2 riders' watts/kg. If that's not the case, the team could be better synced. IMO. It makes a huge difference if both riders have the same power profile around the pedal circle. That tandem shouldn't be flexing.