It's a problem. Rotors are very low mass and heat quickly. Rims have a much larger mass but overheating can blow a tire, which is also not good. I have rim brakes and there are just certain descents I don't do on the tandem, though they are no problem on a single. If I have to do steep descents, I put on the rear wheel with the drum brake. We can handle anything with rims and the drum.

There was a recent huge argument about this over on the touring forum. Clearly there are people who have had stopping problems and got religion and there are those who have not. A riding buddy with a BB7 rear disc smoked the disc and went off the road on a rather short but very steep descent. His front rim brake failed entirely, his fault, but now he won't ride the steeper ones with me any more, even if I only have rims. Caution is good.

Our tandem has a more even weight distribution than a single but the long wheelbase decreases weight transfer. I brake about the same front and rear because it's much more of a heating problem than a traction problem. Other people's experience on that?