Preheat oven to 350 degrees F (175 degrees C). Butter a 9x13-inch baking dish.

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.

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.

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.