Just another perspective from from a Colorado tandem - we have a rear disc brake on both of our road tandems and have at least 10,000 miles riding in the Colorado mountains. We're about 270 lb team and have also done loaded touring (aproximately 100 lb bike with gear). Never any problems with the rear disc brake. As mentioned previously, it is key NOT to ride the disc brake. Alternate between the front and rear brake and also let off the brakes entirely on straight roads - this will give your rim and rotor some time to cool down.
We spend most of our time on the mtb tandem now, but are always up for riding with other local tandems. Hit us up if you ever want to ride.
Enjoy the new bike!!!