Chopping Block Table

I plan to make a 20" x 40'" chopping table. I can get two 20 x 20 x 1 pieces of edge grain (not end grain) maple chopping block for about $40. However, I would like the table top to be at least two inches thick for pounding strength.
Will screwing a 1" plywood sub-base to the bottom of the chopping block through an elongated groove in the plywood protect against the chopping blocks movement? How long should the groove be?
How tightly can the pieces be screwed together and not defeat the slots' purpose?
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