Preheat oven to 375°F (190°C).
In a large bowl, combine peaches, blueberries, sugar, flour, and lemon juice. Toss to coat.
Pour fruit mixture into a 9x13 inch baking dish.
In a separate bowl, combine flour, oats, brown sugar, and cinnamon for the topping. Cut in the cold butter using a pastry blender or your fingers until the mixture resembles coarse crumbs.
Sprinkle the crumble topping evenly over the fruit.
Bake for 35-40 minutes, or until the topping is golden brown and the fruit is bubbly.
Let cool slightly before serving. Serve warm with vanilla ice cream, if desired.