For safety reasons, I would insist on having two different kinds of brakes based on two completely different technologies. That's a basic lesson of reliability engineering. If one brake fails for any reason, you want to make sure that your other brake(s) won't fail for the exact same reason. On our tandem, we have two rim brakes and one disc on the rear wheel, but never experienced any serious brake failure. If we had to ride a tandem with discs on both wheels, we would feel safer with at least one extra rim brake, just in case the two discs fail for some reason.