A few thoughts:
1) I don't think the age/mileage of the wheels had any effect on this. The brake tracks showed virtually no wear prior to this incident. The fact that we got 20,000 trouble free miles out of them indicates to me they were plenty strong, and well suited to our use, other than this heat issue.
2) I do think the TRP/Spyre hybrid disc brake will help. While more braking force comes from the front, as Lichtgrau points out, the weight distribution on a tandem lessens that effect. The Bengal brake we were using could not even come close to locking the rear wheel. The TRP can lock the rear, and more importantly, modulate just short of that.
3) Carbon fiber clinchers have gotten better in the more than 8 years since ENVE Classics were designed. The current popular recommendation for go fast tandem wheels appears to be Zipp 404 Firecrest rims with 28 spokes. (Calfee's current recommendation). According to Zipp they have never had a heat related failure on a 404 Firecrest.
4) Future setup: for most of our riding, other than in the mountains, we'll set the bike back up with rim brakes and Zipp 404's. The bike weighs 24lbs, and I don't want to add several pounds with discs front and back. For the occasional ride with big descents, I'll put the TRP disc on the back, and an older Zipp 808, with an aluminum brake track on the front.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.