I'd pick disc brakes for my next switchbacked descent on a loaded tandem. They fade before anything catastrophic happens. When you feel the fade, come to a complete stop and let 'em cool.

That said, I've only stopped to let rim brakes cool on the tandem once in 8 years of riding it. We were on a dirt mountain pass with 4 full panniers. I think the GVW was 400lb. The descent was Gibbons Pass, heading north into Sula, MT -- 2000ft over 7 miles with a whole bunch of switchbacks. I wasn't worried about a blowout, but the brakes were fading a little bit before we pulled over. We had cantilever brakes and well-worn Bontrager Clyde rims.

A few years ago, we got a new fork and "upgraded" to a BB7 road with 203mm disc in the front. The rear is still canti with a Dyad rim. I haven't felt it fade yet. None of the descents here in New England are long enough to justify brakes with more heat capacity.

If you're stuck with rim brakes and want something that can handle the gnarliest descents, pick a heavy rim, such as the Velocity Chukker.