[dripton]

You need a rear brake for two reasons. One is redundancy: if the front brake fails, you want another option, even if it's not quite as good. The other is spreading the braking heat over twice as much surface area on long fast descents. (Some tandems have a third brake for that reason.)

[Unman]

The redundancy point is true. The tandem drum brakes are less to do about heat dissipation than stopping your hands cramp up on a long descent. Often you just actuate the rear drum to provide constant drag on a descent.

[Unman]

For anyone interested, specialist tandem outfitters used to shave off the heat dissipation fins on old-style Arai drum brakes. The point of said brakes was not to dissipate heat, but to stop the tandem. It is true that very old tandems with rim brakes had heat-dissipation problems.

http://www.bikeforums.net/tandem-cycling/52661-shaving-arai-...