Preheat oven to 350 degrees F (175 degrees C). Butter a 9x13-inch baking dish.
Step 2
Mix apples, rhubarb, 1 cup white sugar, 1/4 cup flour, and 1 teaspoon cinnamon in a bowl until thoroughly combined; spread fruit mixture into bottom of prepared baking dish.
Step 3
Mix quick oats, 1 1/2 cup flour, and brown sugar in a separate bowl; cut butter into the oats mixture until crumbly. Spread the oats mixture evenly over the fruit mixture. If desired, combine 2 tablespoons white sugar with 1/2 teaspoon cinnamon in a small bowl and sprinkle cinnamon sugar over the top of the crisp.
Step 4
Bake in the preheated oven until the fruit is bubbling and thick and the topping is golden brown, about 45 minutes. Let cool 10 to 15 minutes; best served warm.