I was under the impression that there was only one layer of PCB's in both the SNES and Genesis... but then again I have never cracked either of them open.
"Being a teenager is to experience a renaissance of the body, mind and spirit. It is to experience life at it’s fullest." - Cliff Eiffler
Well, they have at least 2, the top layer and the bottom layer, but I think they might have a middle layer.
I say that because I cut part of the ground strip off of the side of my snes and it stopped functioning after that.