On the subject of overheated rims (leading to possible blow-outs) with rim brakes, I recently toured Provence with my wife and daughter and my touring bike had very old Rigida Grizzly CSS tungsten carbide impregnated rims, intended to reduce rim wear (which they do, practically to zero) and to dissipate heat. On a few very long descents with lots of braking I had to stop and wait for the others, and I took the opportunity to touch the rims to judge the heat build up. They were barely warm. The wife's ceramic coated Open Pro rims were also quite cool, but my daughter's plain alloy rims were hot.
The CSS Grizzly rims do seem to deal with the heat problem. The drawback (apart from the price) is poor wet weather braking for the first few thousand kilometres of use, even with the recommended Swisstop pads, but I have been told that Koolstop Salmon pads deal with this. My rims are now worn in and work well with any pad. No rim wear, very slow pad wear, and good effective braking with my cantis.
This is all on a tourer, not a tandem, of course.