I'm not so sure the PC gets the second AoO. You normally don't get an AoO because someone picks the charge action. Check out table 8-2 in the PH. Charge action says No AoO.
But, someone charging may provoke an AoO because of the spaces they move through. Usually this is what happens when you charge someone with a reach weapon. They get you when you move out of the space you can reach 10' away.
The way I read Hold the Line, you don't get an AoO simply because someone chose the charge action. I think you get it because they are moving into a space you threaten (rather than when they leave it) while they are charging. And since, even with Combat Reflexes, you only get 1 AoO against an opponent for each type of provocation, I think you get only 1 AoO.