I have some regulars and when I want to visit their place of work but want a different girl I don't book in advance. 9 times out of 10 she will be busy when I get to the shop and I have the choice of another. If she is available I will go with the regular and make up some lame excuse why I didn't pre-book.
If she was busy when I arrived and took another girl and happen to see her after the session I just tell her I took a chance that she would be free and didn't have time to wait for her to finish her previous booking.
Most ML's don't mind you seeing other ML's