To echo the above - having a Plan B doesn't mean simultaneously contact two SPs and choose one preferentially if they both get back to you. It means, if Girl A isn't available, then try Girl B. Messing SPs around like that gives you, and by association the rest of us, a bad name, which in turn encourages them to up the ante with deposits, AW feedback, and all the other speed-bumps to having an uncomplicated shag. Please don't do it.
What sometimes happens is I contact Girl A, radio silence, I shoot out a hopeful message to Girl B, and then both A and B reply in quick succession. What to do then? I just judge it on a case by case basis weighing up wheree I am geographically and how much I want to see each. Possibly I will then prioritise Girl B as I can justifiably tell A "sorry, i didn't hear so made other plans", I still feel a bit guilty whatever the decision though. As another poster mentioned, a walk-in parlour makes a suitable Plan B.