I'm maybe pathetic, but I would have clarified before I fucked girl 2. I know it risks ruining the moment, but 30 min later when they ask for the other payment it would be just awkward. They imply that they thought you understood. You share your total misunderstanding. Of course you can walk out without paying extra, but you risk ruining a next time with girl 1 (and if seen her for 3 years, that seems a waste).
If it is a freebie, they just say 'no silly she just wants to, shut up and enjoy'. Fifteen seconds of small awkward is better than serious awkward later.
But then I have been to clip joints when I was a lad, and I know frightening awkward. Always clarify. If she wants to fuck you for free, checking that she wants to will not stop her.
Anyway, next time you book, ask if Girl2 can join. Ask for what you want. Pay what is asked. Make it all clear. Businesslike on the phone and text. Crazy in the bedroom.