Heat oven to 350ºF (325ºF for dark or nonstick pan). Make and bake 1 cake mix as directed on box for 2 (8-inch) round pans, using water, oil and eggs. Make and bake second cake as directed on box for 13x9-inch pan. Cool cakes in pans 10 minutes; remove from pans to cooling racks. Cool completely, about 1 hour. Refrigerate or freeze cakes 1 hour until firm.