Hm, the problem here is that damage doesn't ever "stack"- only bonuses stack. The stacking rules were implemented to make the mosh of old rules about what protective item works with what make more sense. I don't think there's ever a reference to damage "stacking" at all. "Stacking" applies to bonuses only, imho.
Now, I'll grant you that the definition of "stack" in the PHB doesn't really spell out the intent on this; but if you ask me, if your sorcerer takes acid fog as one of his few precious spells known and then can only effectively hit you with it once per combat, he's been ripped off. I see no reason why a more intense acid couldn't do more damage. Again, if you use Melf's acid arrow as a comparison, if you shoot that lich over there with three acid arrows he takes three times the damage that he'd suffer from one. Acid fog is like a low-damage fireball that hangs around, not like, say, prayer or bless. I mean, if you get caught in four fireballs in one round, you take the damage from each one, right?