Hi,
I am an experienced (old) tandem rider with 10 -12 years, lots of multi day and weeklong events, and single bike racer.
I would strongly suggest a disc brake if you are riding anywhere with long or really steep down hills, or if your team weighs a lot. I always say both people, both incomes, both busted up is best avoided. If you are steering for 2 be careful and don't use any suspect equipment.
You can over heat a rear rim and blow your tire on long descents in the mountains. I have done it and luckily did not crash. There are braking techniques that can help prevent this, alternating front and rear, pumping the brakes to slow the bike and not continue to apply the brake for a long time or stopping and letting the rim cool half way down. These may work, but if they don't, consider the blow out happening.
I currently have the avid BB7 on the rear wheel and regular dura ace rim brake on the front. In the past I used a formula disc brake on a different tandem, it had a remote master cyclinder and was cable actuated. The formula had more stopping power and I never had problems with it other than one time it needed bleeding. Many people had problems and some failures and vowed to never use that brake again.
There are some new brakes out from TRP, one hydraulic, one mechanical. These are just not being put on tandems (it voids the warranty) and I may buy one of them after some more reports.
There is a tandem bike forum that has a long thread on the new TRP brake and the people using them the last month are starting to give updates, but not much yet.
It would be important to know their team weight and descents they are doing and some of the people include that info. I only mention this because of downhill and gravity.
This forum has a few people that really know their tandems and provide great info.
Tandem Cycling
rob