Preheat the oven to 375 degrees F (190 degrees C). Lightly grease a 9x13-inch baking dish.
Mix rhubarb, peach, white sugar, and ¼ cup flour together in a large bowl until evenly combined; pour into the prepared baking dish and spread into an even layer.
Stir brown sugar, quick oats, and 1 ½ cups flour together in a separate bowl. Cut butter into flour mixture with a knife or pastry blender until mixture resembles coarse crumbs; sprinkle evenly over rhubarb and peach mixture.
Bake in the preheated oven until golden brown on top, about 45 minutes.
