The smart thing is not to get involved with somebody who has a boyfriend and not to expect other people to want to get involved with you when you have a girlfriend. The first girl you mentioned isn't what you want? You want to be available if somebody you like comes along. Then leave her, and make yourself available. Don't string her along.
Now the second girl also has a boyfriend. It shouldn't matter that it's long distance. He could come over or she could say something at any moment, and you would get your heart broken. The more you let yourself get involved with her, the more you're setting yourself up to get hurt. Whatever this event is, I doubt it's worth the complications of a broken heart. I would let her know that you're not yet ready to handle being just friends. Hand your ticket over to her and let her bring somebody else. You'll lose some money and miss the event, but it's better than the alternative.
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy