Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Mix rhubarb, strawberries, and 1/4 cup sugar in a bowl; spread fruit mixture into prepared baking dish and sprinkle remaining 1/4 cup sugar on top.
Sift flour, baking powder, baking soda, and salt together in a bowl. Beat 2/3 cup sugar and butter with an electric mixer set on medium speed in a large bowl until fluffy, 4 to 5 minutes. Beat egg and almond extract into butter mixture until incorporated, about 3 minutes more. Pour in the flour mixture alternately with the sour cream, mixing until just incorporated. Batter will be thick.
Spoon batter evenly over rhubarb mixture.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 45 minutes.