I have an observation that leads to a question that the manufacturer may have to answer. How far back on the tandem can the center of mass be moved without changing the handling and braking characteristics of the tandem? I ask this question since you mentioned the readjustment of the shock absorbing stoker seat post to accommodate the heavier rider. It is my understanding that tandems are engineered to assume that the heavier rider will be the captain. This sets the center of mass more forward and I suspect sets design criteria for the frame, fork, wheel and component selection. If one moves the center of mass back toward the rear wheel, the handling and braking of the bike may be different. The reason I point this out is that you may want to experiment a bit to see how the bike handles and brakes under different conditions before you descend a steep hill at high speed come to a sharp turn and crash the bike. I suspect the normal operating conditions will not be a problem. The same situation applies to rear engine cars. One has to drive them differently e.g. applying power through corners.

As a second point, the rear tire pressure will have to be increased when the heavier rider is the stoker.

Good luck