Why do people commonly refer to Na2HPO4 as Sodium Hydrogen Phosphate, rather than Disodium Hydrogen Phosphate?
Well, turns out that I needed to look at this a little differently.  I looked at calcium hydrogen carbonate and then realized that hydrogen is part of the polyatomic ion.  It's a bit harder to recognize when there are no parenthesis, like in the case of sodium hydrogen phosphate, but the same thing applies there.  Therefore, the name, sodium hydrogen phosphate is correct because there are still only two distinct ions in the compound.