It's a common scenario - the law of sod if you like - the key thing is to try to handle it with a degree of elegance.
Option 1 is to stick with boy A - after all, you had a good time, and whilst boy B might look gorgeous, he might be a waste of time between the sheets.
Option 2 is to abandon ship and go somewhere else that evening - check bin, and if boy A looks puzzled, make out you're not feeling too good and are turning in early. Go back a couple of nights later, by which time boy A has probably been bedded by someone else and his expectations will have become history.
Option 3 is to entertain both boys together in the bar and be quite generous when it comes to buying them drinks. Boy A will have his tip money from the drinks and you won't have cut him dead. Unless he's very green to the scene, he'll be well aware that most punters are butterflies and that you might well take him another time.
Option 3 is the most expensive solution, but is the one I'd normally opt for. While you're sitting together, Boy A will also probably tell Boy B what your taste in bedroom gymnastics is. If Boy B doesn't fancy that, he'll leave you with Boy A and you'll be spared a frustrating session. Option 2 is my preferred escape method when I find myself with two people I've previously been with who are vying for my attention..