I've read that after torting a 12 x 18 sheet cake, you should cut the top layer in half to make it easier to take off of the bottom half to fill. I have a large cookie sheet with only one lip that would lift the whole top piece so I'm not sure that cutting it in half would be necessary. Any thoughts or advice?

I never cut them in half. I do what you do. Either a cookie sheet or a cake board.