At what point is filling doubles two fills?

[Chris H]

We've briefly touched on the cost of fills in another thread, so back to my original question. I have low pressure double 80s. If I bring them in to get topped off from 1500psi, should the charge be for one or two fills?

Let me just add the discalimer that this isn't directed at any specific LDS, but meant as a blanket question to see what others think.