maximum reverse voltage around diode
Real world answer: 100 V on each, but be sure to choose a diode rated for substantially higher than that, say 200 V.

Answer your professor wants: A few volts more on D1 than on D2, because part of the leakage voltage through D2 can't pass through D1...therefore it must go through R1, which increases the voltage across R1 (which is the same as the voltage across D1).