There are so many factors. Financial: If you were doing double sessions, would you do as many regular sessions? Focus: If you are focused on yourself it can be more fun, more things happening to you. If you are focused on the mistress either you don't get enough of her, in an unbalanced session, or you might think you are missing on both if the session is balanced. Which is why I think if you do a double session you should record it as well.