I've heard of two occasions when this happened.
The first time, one guy forgot he had booked two escorts at the same time. Sometimes that's the danger with pre-booking, especially if the john sees a large number of girls during a trip. What happened is that the first girl arrived & as they sat and got to know one another better over a drink (or a joint), the second girl knocked on the door. The guy had totally forgot he had double-booked. Since both girls were fairly attractive, the girls decided to stay & did a duo with the guy. They actually had a great time, from all accounts.
The second time it happened, a girl who had met the john offered to do a duo with a friend of hers, who happened to work at a different agency. In other words, the second girl came on her own time (outside the agency setting). The guy didn't mind the arrangement since it had been offered to him by the girl anyway. He felt no loyalty towards the second girl's agency.
Speaking of duos, i have a birthday coming up this summer & i may be in a mood for one, not having had a duo in 3 years. Since it'll be a very special birthday, i may be in the mood for something different. Maybe a trio. We'll see....