I made mine that way.
used one sheet of 18mm and one sheet of 9mm.
Put the big sheet on top, scored the entire mating surfaces of both, and glued.
Clamped them together, and then screwed from the bottom into the top every 30cm (12") in each direction.
I left it overnight for the glue to set real good.
But it pays to make sure the top sheet surface is totally flat before you start, its not guaranteed that it will be, and if there are any waves in it, youll struggle to get good cuts on short pieces of wood.