Ooops! I had trouble getting my 12x18 cake to cook completely in the middle last night...but it eventually got done. I wrapped it up in plastic wrap and stuck in the fridge overnight to firm up...took it out earlier, unwrapped it and noticed that the edges were really crisp, or hard. I'm thinking, since I have another to go upside down on top of it ( with bc in between) that I could just shave the edges off with a knife? Would that make any difference when I go to decorate, since the bottom layer will be right side up and the top layer upside down? TIA

This is a perfect example of when a simple syrup comes in handy. If you cut your edges you run the risk of getting a lot more crumbs. If I understand you correctly though the crispy parts will be on the bottom? Even better! Brush them with the simple syrup, let it sit for a few mintues and you will be good to go!

Are you taking about those little crispy bits around the top ridge? Just cut them off with scissors, much easier than a knife!

tripletmom - yes, the crispy edges will be on the TOP of the BOTTOM layer...tht make sense?? lol Instead of torting one 12x18, I'm using two individual 12x18 sheet cakes... Did this because even though the pan is 2 inches deep, the cakes don't actually come out 2 inches deep, at least with me they don't! lol Just want to make sure there's enough cake.
I'd thought about the simple syrup...so it will soften up the edges enough?

I can only speak from my experience and that is what works for me. The buttercream adds a certain amount of moistness however I find the syrup adds what the baking process took out. Brush it lightly and it should be perfect once again.

JennT, anything 12" or bigger, I use an inverted flower nail in the center to help distribute the heat. Your edges got overbaked b/c the center had to cook longer, so they're crispy. You can trim them off with a knife if you don't want crispy edges in your cake. Crumbcoating the sides or icing with the icer tip will help with the crumbs.