How do I get vanilla loot tables to work with slabs

I can change vanilla loot tables through a procedure or a /summon item procedure but whenever you change one of a slab, double slab blocks only seem to drop one item as well.

For example if I changed the loot table of polished diorite slabs to regular diorite slabs, it works fine. But when there are two stacked polished diorite it seems to only drop one regular diorite slab