Coat a 10-inch square casserole with cooking spray. Arrange 3 cups sweet potato and 1 1/2 cups apple in casserole; sprinkle with 1/4 teaspoon salt. Combine syrup and margarine; stir well. Drizzle half of syrup mixture over potatoes. Repeat procedure with remaining ingredients. Cover with plastic wrap; microwave at HIGH 10 minutes, rotating a half-turn after 5 minutes. Let stand, covered, 5 minutes.