Best downhill braking method to avoid overheated rims?
The power output from braking is linearly dependent on speed and braking force. To reduce heat buildup, you have to reduce either one of these. The options are:


Descend very slowly. This is boring, but works.
Descend so fast that air resistance takes most of the force (the force depends roughly on second power of speed). This is the idea behind the advice to brake sharply for corners: You go faster between them, which lets the rims cool.
Increase your air resistance. A big flappy wind jacket slows you down by a surprising amount.
As you already found out, dividing the load between two rims helps.


Too hot to touch isn't necessarily too hot for the rims and tires. Also, one thing to do is to reduce tire pressure before big descents, the temperature will increase it back. Jobst Brandt's posts on descending also touch the subject: http://yarchive.com/bike/descending.html