You can always ask when you go into the shop who will be doing the massage just to be sure, especially if it's a new shop.
Worse than this is when you actually get the pretty girl and when you turn over and she asks if you want extras she says "well I don't do extras but I will get the other girl for you" - there is no way out of that situation that I know of other than to say "yeah me either, please continue the massage".