The general rule in braking is that the size of the wiped area is what matters most. So we can disregard the size of the shoes and focus on the difference in the rim and disc, where the heat will end up.
OTOH rims have far more ability to uptake the heat energy, by virtue of the the material, surface area, and volume. They also have more ability to transfer that heat back to the surrounding air. But here's where it get complicated.
Rims are limited in the maximum temps because they are in contact with rubber tires, whereas discs can operate at far higher temps. So you'd need to compare the energy needed to heat the rim to X, to what's needed to heat the disc to the far higher temp. Then because higher temps increase the rate of cooling, that has to also be factored.
Tossing all that into a hat, and comparing based on some experience with brake heating on hills, I suspect, but can't confirm that rim brakes will fare better. In any case, I suspect that both systems will be inadequate over a long descent on a tandem, so riders will have to either accept riding closer to terminal velocity or, as truck drivers do, stop and cool their brakes from time to time.
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FB
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Last edited by FBinNY; 09-13-17 at 08:50 PM.