Preheat oven to 350°F. Gently stir together peaches, blueberries, cornstarch, lemon zest, lemon juice, and 1/4 cup of the granulated sugar in a large bowl. Spoon mixture into an 11- x 7-inch (2-quart) baking dish.
Whisk together flour, brown sugar, salt, cinnamon, and remaining 1/4 cup granulated sugar until mixture is completely smooth (no clumps). Add butter, and toss to coat. Using fingers, rub butter into flour until a pebble consistency is reached and flour is fully moistened. Stir in oats. Crumble topping evenly over peach mixture.
Bake in preheated oven until top is golden and fruit is bubbling, about 45 minutes. Serve warm.