yeah, I would 100% recommend that you try to at least get to know the other girl. this kind of puts you in a bad spot though.
- If this "double date" is your first date with this girl, make DAMN SURE - I CANNOT STRESS THIS ENOUGH - you spend most of the time talking to her and listening to her. try not to look at, think of, and above all COMPARE this girl to the one who's dating the other guy. If you fail at any of these things, the girl who likes you will take it as a great insult, and it's just not cool to insult the ladies.
- This "double date" is obviously a two-handed ploy by the girls. First off, to say once and for all that the girl you love doesn't like you, AND a secondary distraction tactic by getting her friend hooked up.
- I'd just try and switch up your brain a bit for now - whenever you think of the girl you like, try and shift your brain to think about the things you like about the other girl.
basically, this is a great opportunity for you to get over this other girl.