The tandem I owned had a drum brake on the rear wheel as a third option, along with a hydraulic rim brake on either wheel.
All three brakes were controlled by the captain in front only.
Normally you'd have your convention brake levers/brifters running the rim/disk brakes, and the third brake would have a control that was not immediately to hand.
On mine it was a bar-end shifter that you could move and leave at a setting without having to hold it. IE there was not enough spring tension to release the brake, the lever held the brake on. For this reason I suspect any drum brake would be cable actuated only, not hydraulic.
I hadn't found a situation where the third brake was needed, until I was solo-riding down a grade and a hard braking effort raised the rear wheel off the ground. Moving my own body weight backward was insufficient. The rest of the descent was a lot more restrained !
As for overheating, the drum brake can take a fair amount of heat before it gets too hot to work well. And then consider it is only there as a limiter, and your main brakes will be cool and ready to stop the bike.
