I'm glad I found this thread as I've been ruminating over this today.
I've just been faced with this and I believe the enquiry is genuine. Having read through the thread, I'm going to offer a slight discount but as it will be a fully interactive 3sum, I am definitely charging for 2 people.
Since logistically, I cannot see two people back to back (as in hours), I've decided to offer a ?20 discount off my hourly rate per person, which still leaves me a healthy profit. (?220) I know I'm going to offer them a drink first (it's an incall on tour) and that 15 minutes will likely be spent on this but I will make it clear that there is 15 mins of extra time included.
I am also going to offer an extended package for 2 hours for ?300 and see if they bite on that one as it will be better value plus we can have more social time.