How do you plan on using the tandem? If your just going to use the bike for day rides the disk brake may be fine. But if your going to tour on the bike you would do better with a drum brake on the rear.
Disk brakes work well to stop a bike, but if used to control the bikes speed on long downhills they over heat and fail quickly. Using rim brakes to control your speed down hills will cause the rims to over heat, leading to tire failure. You could try "pumping" with rim brakes to control your speed, but it takes longer to dissipate the heat built up in the rims then it dose to heat them. So, each time you apply the brakes the rims heat up a little more. Trust me having a tire fail at 30 mph is sure to make an impression on your stoker.
The drum brake can better stand up to the heat when using the brake to control your speed. 50 mph comes up real fast on a tandem. Yea, it's not high tech, its not light weight at three pounds, but it works.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." - Arthur Conan Doyle