In large bowl, make cake batter as directed on boxes. (Both boxes can be made at one time; do not increase beating time.) Spread 2 3/4 cups batter in each 9-inch round pan. Spread 1 3/4 cups batter in each 6-inch round pan. Bake 9-inch pans 24 to 29 minutes, 6-inch pans 22 to 27 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes. Remove cakes from pans. Cool completely, about 1 hour.