Preheat oven to 425°F. Pour both cans sweetened condensed milk into a pie plate. Cover plate tightly with foil. Place pie plate in center of a rimmed oven-safe pan or baking sheet. Pour boiling water into pan, enough to reach halfway up the sides of the pie plate. Bake until milk is thick and golden brown and becomes toffee, about 2 hours, adding more boiling water to the pan every half-hour to keep it at the halfway mark.