I often make double layered sheet cakes. I usually make homemade cakes instead of cake mixes. They usually cook flatter. Cooking at a slightly lower temperature (325) for a little longer will also not let the cake dome up as much. Before I put the batter in the pans I always line the pan with waxed paper. This stops the cake from sticking and helps hold it together when you remove it. After flipping over the cake when it is in place just peel off the waxed paper. Good Luck!