Way back when, have pedaled a 116" gear (56x13) at 43.5 mph . . . both of us bouncing on the saddle; as the only mixed tandem couple and not willing to give an inch to several male/male much younger tandem duos on a slight down hill run from Oracle to Oracle Junction, for 11 miles.
Those were the days . . .
Have test ridden a prototype tandem (solo) with front/rear discs on downhill and hit the brakes at 30+ mph. Loads of 'stop 'n skid' power but considered it overkill.
Prefer our current setup, Dura Ace front calipher + Tektro Mini V-brake rear which provides all the power we need for downhills/traffic situations. Can ride this combo solo, hit just the front brake and instantly pop up the rear wheel. Feel quite safe with this brake setup.
Just our experience/opinion.
Pedal on TWOgether!
Rudy and Kay/zonatandem