Hi Dave.
That's a happy problem. You have two choices.
1. Flake on Girl A the last minute. This will increase her attraction to you. It's called triggering her intrusive thinking. Just think of an incident when a hot babe flaked on you and you kept on thinking about it for days on WHY she flaked on you. That intrusive thinking made you WANT her more. If women can do it to men, men can do it to women, eh? Give Girl A the gift of this emotional pleasure in your repertoire of emotional roller coaster ride techniques.
2. Go on with the date with Girl A. This has two advantages. You'll show social proof which will demonstrate your high value and you'll give an emotional roller coaster ride to TWO girls instead of just one. Moreover, Girl B's friends will market you as a bad boy. They'll discourage Girl B not to date you anymore because you're NOT a nice guy and that you're a BAD BOY. Bad boys usually get the chicks, right?
The bad boy routine is to show that while you're an asshole, you have a soft heart inside. Say something like:
Girl B Routine
You: Hey, I've made a COMMITMENT to my best friend to go out on a date with his girlfriend's bestfriend. I'm very LOYAL to my friends. (You can change the excuse but don't forget to mention the words "commitment" and "loyalty".)
Girl B: Fuck you asshole.
You: I understand how you feel.
Ask her next time to a date where there are many children. Then show (not tell) her that you love children.
Girl A Routine
You: Hey, I want to introduce you to a friend that I've been dating just recently. I met her at an orphanage (or whatever). We took care of puppies/children/panda bears/stray cats (whatever) during the event. She's right there. Come.
Girl A: Okay.
You: Hello. I want you to meet a very wonderful person whose been saving the dolphins in the Sahara Desert (or whatever). My business partner and I wanted to scout for a charity fund for some tax breaks.
Leave the introduction vague as to who is involved in the charity or create your own routine.
Girl A might get into a fight with Girl B. Or maybe both girls will grab your balls and slam them on the table.
Just look at it though from a positive point of view. Girls love drama.
