I often bake two layers at the same time. The first one goes on the top shelf towards the back and the second one on the lower shelf towards the front. Halfway through, I rotate them. The top one comes forward and the bottom one goes toward the back. Sometimes I have placed them both on the same shelf in the same manner. I still rotate them, front to back, back to front. If I place them on separate shelves, I don't like to pick them up and change shelves. Since the top shelf is usually hotter than the bottom (heat rises) I will remove that one first when a toothpick comes out clean. I leave the bottom one in for about 30 seconds longer. I have never had a problem. My oven is electric.