I have not ridden a tandem bike and when I worked in a bike shop never worked on one, so maybe I am wrong here, but it is my understanding that the purpose of a drag brake on tandems is for those long downhills where a solo bike brake system would be overwhelmed. Thus, it is a third brake, only applied for those situations where you need it. Was mounted on the non-drive side rear hub on bikes with rim brakes.

On my loaded solo touring bike going down Going to the Sun Road westbound for a drop of several thousand feet, I stopped twice to check how hot my rims were and one of those times waited for the rims to cool before I continued. That route is about 6 percent grade, so not that steep but it seems to go on for a very long time.