Preheat oven to 375°. Spray an 8-inch square baking dish with cooking spray.
In a medium saucepan, whisk together ½ cup granulated sugar, cornstarch, cinnamon, and orange juice until smooth; bring to a boil. Cook, stirring constantly, until thickened, about 2 minutes. Remove from heat; stir in peaches and blueberries. Transfer to prepared dish.
In a medium bowl, whisk together flour, remaining ¼ cup granulated sugar, brown sugar, baking powder, and salt. Using a pastry blender or a fork, cut in butter until crumbly. Pour in buttermilk and stir just until dry ingredients are moistened. Evenly spoon dough over fruit mixture. Sprinkle with sanding sugar.
Bake, uncovered, until topping is golden brown and filling is heated through, 40 to 45 minutes.