Preheat oven to 425 °F
Peel and cut peaches. I blanch the peaches to remove the skin easily. I sliced the peaches and then cut the slices in halves or thirds.
Cube the mango. I cut around the core of the mango and then sliced.
In a large bowl, combine peaches, mango, granulated sugar, brown sugar, cinnamon, lemon juice, and corn starch. Mix to evenly coat and pour into a large baking dish. (I used a casserole dish about 9x13)
In another large bowl, combine flour, granulated sugar, brown sugar, baking powder, and salt. After mixed, blend in the butter with your fingertips, a fork, or a pastry blender. Mix the butter in until a coarse meal.
Stir in boiling water to flour mixture and mix until just combined.
Pour batter over fruit mixture by spoonful drops. You can sprinkle the batter with cinnamon sugar before baking if you want.
Bake until the topping is golden brown, about 35-45 minutes. The fruit should be bubbling as well.