I'm going to sum this up as simple as possible.

There's a girl i met at a friends party(this is when i first started to get in the game). I became good friends with her BFF, and i have her friend's trust. Now the problem is i asked both of them to hang out before but they got stuck in traffic yesterday. So it was a no go. Now they go back to college(they dorm), this sunday, but i want to make a move before that. I don't want to ask them out again b/c of the cat-string theory. So how can i get them to hang out without asking them personally?

The only idea i have is to ask another friend of mine, who goes to school with them, to to hang out and for him to conveniently invite them. From there on i think i know what i should do.

But my question is:
If my idea backfires, what's another way i can ask the girl(her bff WILL come regardless) to hang out without coming off as needy

Fuck the whole cat-string theory.
Be genuine, and ask them to hang out again because you wanted to before they go back to college. There is nothing wrong or AFC about that, you just simply enjoy their companies.
But make sure to make it in an interesting alpha invitation. Don't just say "hey can we hang out this week?". DON'T ASK, TELL them to do it.
If they still try to bail out then their level of interest isn't high enough, you need to move on or build more attraction (which is a less preferable option when you only have till weekend)
It's also a way to validating their BS, if they really couldn't make it last time, they will be more than likely agree to meet up. But if they were making excuse, they will do the same this time.

Thanks, that's really useful