Chances are, if you're not a very large team (and clearly you and Kay have always been welter weights) or don't push road and off-road tandems to the ragged-edge where high brake demands and heat loads truly get generated, then you would probably not have much experience with brade fade. From my '04 Survey of Hobbes, BikeForum/Tandem & Double Forte readers, this was a pretty good representation of the bell curve on team weights. Again, I suspect you and Kay are way up on the top / light end of this scale while the OP is clearly on the lower / heavy end.

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As for the masses, Question #8 from my 2008 Survey of Hobbes, BikeForum/Tandem & Double Forte readers, (extracted image posted below) explored this subject and the number of affected riders is probably about right at the macro level: http://www.thetandemlink.com/surveys/fallsurvey_08a.htm

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From personal road experience, we've had partial brake fade with both rim and disc brakes several times during extremely hard braking ahead of switch backs on very high-speed descents, , e.g., dropping from 55 mph to 20 mph in 1/8 to 1/4 mile really cooks your brakes.

Riding off-road, we've melted the rear brake pads off a set of V-brakes as a result of total brake fade on a long, steep fire road descent, immediately after which we upgraded to a new full-suspension tandem with hydraulic, open-system downhill racing discs. We've successfully cooked those downhill brakes to the point where heat expansion caused the rear disc to lock-up on several occasions and have experienced several partical brake fade episodes on some of the faster, technical descents.