Well Dazed, it doesn't seem like you are comfortable with what you did with girl #2 at the party.  And yet you think you will be comfortable making a date with her and making an effort to be alone with her.  As a friend.  Uh...I'm a little uncomfortable with the logic here.  It reminds me of new age TV commercial logic.  Anyways, I know this is a novel idea, but have you thought of

He doesn't deserve either of them

discussing your feelings/relationship with your current girlfriend/lover?

Ok, this really is not as easy as I am presenting it.  It is always tough to discuss serious relationship topics when you yourself are not sure of your feelings.  And when it can affect your future/life.  But I feel that this is what a mature person would do--respect his significant other enough to discuss these important subjects instead of running around behind her back.  After all, the old time-honored question, "what would you want HER to do if your roles were reversed" probably is something you should be asking yourself.