Upon my first visit to my current vet, I was told you should have 1.5 boxes PER CAT. Now I have two 'kids' and three boxes, though one box is out of commission. NO good place to stick it in the new apartment just yet... that and I couldn't get enough of my usual litter to make up all three of them.
If one box per cat is working for someone, then there's no reason to add another. I have the third as 'back-up.'
Speaking of which, I should get to the grocery store. I'll check for litter there. I wish Petco or Petsmart would hurry up and build a store within walking distance of me. We NEED a store close by.