Make the Filling: Toss together the peach slices and sugar in a large bowl and let stand for 30 minutes, tossing occasionally. Drain the peaches in a colander set over a large bowl, reserving the drained juices. In a small bowl, whisk together ¼ cup of the drained peach juice, cornstarch, lemon juice, cinnamon and salt (you can discard any excess peach juice). Return the peach slices to a large bowl and toss with the juice mixture. Gently fold in the blueberries and transfer the fruit mixture to a 2-quart baking dish.
Make the Crumble: While the peaches are draining, you can start on the topping. In a medium bowl, whisk together the flour, ¼ cup of the granulated sugar, the brown sugar, cinnamon and salt. Add the butter and vanilla extract and use a pastry blender (or your fingers!) to work the butter into the flour and sugar mixture. Blend it together until a crumbly mass forms. Transfer the mixture to a parchment-lined baking sheet and spread out into a single layer (the chunks should be about a ½-inch in diameter, give or take). Bake until the crumbles are lightly browned and firm to the touch, about 20 minutes. Adjust the oven rack to the lowest position and increase the oven temperature to 375 degrees F.
Assemble & Bake: Transfer the par-baked crumbles to the top of the peach and blueberry filling, spreading it into an even layer. Break up any large pieces and lightly tamp down on the crumbles so they adhere to the filling. Sprinkle the remaining tablespoon of granulated sugar evenly over the top of the crumble. Bake until the crumble is golden brown and the fruit filling is bubbling around the edges, about 30 minutes. Allow to cool on a wire rack for at least 15 minutes before serving.