Heat oven to 350°F (325°F for dark or nonstick pans). Grease bottoms and sides of two 8-inch round cake pans and a 13x9-inch pan, or spray with baking spray with flour. In large bowl, beat 1 cake mix, 1 cup of the water, 1/2 cup of the oil and 3 eggs with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Pour into round pans, dividing batter evenly. Bake as directed n box for 8-inch round pan. Cool cakes in pans 10 minutes; remove from pans to cooling racks. In large bowl, beat remaining cake mix, remaining 1 cup water, 1/2 cup oil and 3 eggs with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, stirring occasionally. Pour into 13x9-inch pan. Bake as directed on box for 13x9-inch pan. Cool cake 10 minutes; remove from pan to cooling rack. Cool completely, about 1 hour. For easier handling, refrigerate or freeze cakes about 1 hour or until firm.