1Heat oven to 350°F (325°F for dark or nonstick pan). Grease or spray bottom only of 13x9-inch pan. In large bowl, beat cake mix, water, oil, whole eggs, and 2 packages drink mix with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Pour into pan.
2Bake 30 to 35 minutes or until toothpick inserted in center comes out clean. Cool 15 minutes.
3Pierce top of warm cake every 1/2 inch with long-tined fork, wiping fork occasionally to reduce sticking. Drizzle 1/2 cup lemonade concentrate evenly over top of cake. Cool completely, 1 hour to 1 hour 30 minutes.
4In medium bowl, stir 1 package drink mix into 2 tablespoons lemonade concentrate until dissolved. Beat in powdered sugar and butter on low speed. Gradually beat in enough of the remaining 2 to 3 tablespoons lemonade concentrate, 1 teaspoon at a time, to make frosting smooth and spreadable. Spread frosting over top of cake. Cut into serving pieces. Decorate with sour candies. Store loosely covered.