Lay pineapple slices on one end of prepared baking sheet and arrange orange slices on the other end. Sprinkle with sugar. Broil, 4 to 6 inches from the heat, until the oranges are caramelized, about 5 minutes. (Watch carefully to prevent burning.) Removethe oranges with a spatula and set aside. Rotate the baking sheet and continue broiling until the pineapple is caramelized, 4 to 6 minutes more, removing any slices that are ready sooner.