Originally Posted by
Atlas Shrugged
But that’s not what the OP has built. Drum brakes in these applications have the advantage of much more mass and surface area to dissipate heat. They also have the advantage of much larger brake pad surface area. Santana made a disc based drag brake but it was over 250mm and Hope made one which was a vented design like in vehicles. Both were very specialized and had limited adoption in an already limited marketplace.
You will also notice that OP is not in fact putting said drag brake on a tandem.
As far as I understood the system is not yet complete, ie. changes can still be made. Rotors are cheap so getting a thicker one should be a non issue.
It needs to be said that with metallic pads it takes quite a lot of effort to overheat a mechanic disc brake to a point where it fails or even meaningfully loses power.