Preheat an oven to 375 degrees F (190 degrees C). Lightly grease a 9x13-inch baking dish.
Mix the rhubarb, peach, white sugar, and 1/4 cup flour together in a large bowl until evenly combined; pour into the prepared baking dish and spread into an even layer.
Stir the brown sugar, quick oats, and 1 1/2 cups flour together in a separate bowl. Cut the butter into the flour mixture with a knife or pastry blender until the mixture resembles coarse crumbs; sprinkle evenly over the rhubarb and peach mixture.
Bake in the preheated oven until golden brown on top, about 45 minutes.
