What if, you don't pull either of them? Have you thought about that? Do. It's a likely outcome.

I would just go off the bat on the night, whatever feels right at the time. Personally, I would gravitate towards the one that isn't your good friend.

Your good friend may well get jealous anyway, but act cool - then you can pick up on it and tease her afterwards.

This is not 'so fucked up' - it's the sort of thing that happens at nearly every party ever conceived, you've just not noticed it before.

Attend with no expectations, then you won't be disappointed at the end - and you may just have yourself a great time, as opposed to being inside your head all night.