Would it be better with two slabs of steel, one above one below?

Intuitively, I don't think so. The steel is colder than the actual oven, what it does better is transfer heat by conduction into the dough (better than stone at that). That only works if it's touching the pizza.

You would heat the metal to temp before putting the pizza in. Dark metals have higher radiant heat, so maybe cast iron above it would be even better than steel.