I needed a full sheet cake size so put two half sheets together but had several problems. I butted the edges up against each other but once I frosted you could tell where they met in the middle. It kind of sagged a bit. Also, putting two half sheet cakes together makes it too big to sit on a a full sheet cake board. Any ideas?

Did you ice between the cakes? You might just have to put more icing in the middle where you said the cakes are sagging. I've bought the full cake board from the cake supply store and two cakes fit on it.

I have made full sheets and have had one rise different than the other, its usually not more than 1/8 to 1/4 inch but it was there. Usually I will crumb coat it and pop it into the fridge so its firm and then reice it and use the icing to bring the smaller side up to level. I havent had a 1/2 sheet that was way off, but I would probably re-bake it or have one side with filling!

If there is not enough support underneath the full sheet, the board flexes and you will see the seam. I use 2 full sheet boards taped together for sturdiness. When I fill the full sheet I use three board taped with packing tape and then use decorator ribbon to hide the cardboard sides.

What size pan are you using for the half sheets: 11 x 15 or 12 x 18 pan?

How thick of a piece of plywood should I use?

I'm still having trouble fitting two of my half-sheet cakes on a full sheet cake board so it will become a full sheet. My pans are 12x 18 and the boards I can buy around here are 17x25 which means I need to trim in order to do a border. I love these pans because they are professional quality with sharp corners. Do I just need to trim and then does that hold up ok? Or, are there other ideas? thanks

Most of the times if the cake is right up to the edge of the full sheet board, I usually don't put a bottom edge on it (if its transported in a FS cake box the box will take the border off). So, I wait until I take it to the party, remove it from the box and then pipe a border on it with my repair kit.