Fuck that, that's why two threads is a no-no. One will be loved more and someone is gonna get upset and just hearing that pisses ME off. That's some stupid bullshit right there. If she's struggling with RP #2, just suggest dropping it or changing it so it's not so low on her list of stuff to do. You're invested in both and she's not. That's not fair to you AT ALL. There's no reason why she's keeping you on the hook and going at a snails pace. Has the slow second thread been going on for a while or is there any chance she'll get into a better "groove"?
As your friend, I say drop the second one because if they're not into it, it's no use forcing it. It's stupid she agreed to do it in the first place but someone has gotta own up and say it's not working out. Or you need to say she's being unfair and to try harder to give posts the same attention because ya know, adults should be able to communicate issues or at least scale back one RP if the other needs more care and attention.
That's really a shit situation, I can see why you're upset. You took a risk and she's not giving you the same effort and what's worse is that she can't even tell there's an issue.
I mean, if I see an issue with us, I try to resolve it. If you're not happy, I'm not happy and then our RP and friendship suffer.
Adults fix problems. Your partner needs to see there's a problem and fix it or you should go back to one thread if you can. Juggling two is tricky and the other person clearly doesn't get how hard it is to do two with one person, it's like putting more of your heart on the line to make two awesome stories and then one suffers. It shouldn't suffer, a posting routine should develop but clearly it's not and it means you have to put up with more crap and stress/worry/grief and that's emotional crap no one needs. :(
I've had that happen to me so I understand the feeling. I hope you can try to make amends and not deal with that because it's doing you more harm than good. *hugs tightly* If you give me their address, I'll go leave a bag of dog poo in their mailbox cause you know, that's the right answer when you step back and think about it XD