Yes, you can survive twins! I'm proof, our twin boys are 11.
First, you're looking at a discipline issue. It sounds harsh, but that's what it is. She has figured out that she has total control over where and when she "potties". You have to give her "incentive" to not potty in her pants anymore.
Check out child psychologist John Rosemond. He's wonderful and has some good ideas with kids who refuse to potty train. We had a really tough one, and it worked great, with much less stress on me. Here's basically his approach.
1. Unless your daughter has a developmental delay that PREVENTS her (or either one for that matter) from potty training, then it's a power struggle right now, and she's winning. Check with the pediatrician if you want to rule out developmental delay, especially considering their premature delivery.
2. Sit them down and tell them the doctor (Rosemond is a doctor) says it's time for them to potty ONLY on the potty because they're "big girls". Because you know it takes some time (not forever) to learn to do that, you're there to help them.
3. This is how the doctor says they'll do it...When they first wake up in the morning, they'll sit on the potty. After breakfast, they'll sit on the potty, UNTIL THEY POOP! This step is very important. They must stay on the potty, no matter how long it takes to poop. AFter breakfast, their body should be ready for it. Eventually, they will decide that their desire to "be free" to play is stronger than their desire to stay in diapers.
4. The rest of the day, they need to be in cloth pants, or even bottomless (poop and pee running down the legs bothers most kids). Expect accidents and do NOT make a big deal of it when it happens. Take them to the bathroom, remind them where they're supposed to potty, and help clean them up. If you determine they're doing it on purpose (choosing not to potty on the potty), make them do all the cleaning.
One other piece of advice based on my experience using this approach...our son has NEVER had a "regular" potty pattern. He would sit on the potty for 45 minutes to an hour after breakfast. Since other kids needed access to the potty, too, I altered it a bit. After 30 minutes on the potty, if he hadn't pooped, I made him sit in a chair by me while I did some work. I set a timer and made him go back to the potty every hour until he finally pooped. After he pottied successfully, he could get off the chair and play. When he asked why he couldn't play, I reminded him the doctor said he was big enough to not need diapers anymore and had to sit until he pottied. It took about 2 weeks. He was very stubborn, but accident free in underpants. About a month later he had a "relapse" and started pottying in his pants (very frustrating). We went back to staying on the potty and sitting in the chair. In about 2 DAYS he was accident free again.
It may sound harsh, but it was much less stress free than any other potty training method we'd tried. He didn't "like" it, but it was very matter-of-fact, without guilt, yelling, or punishment. He had a choice to make potty and play, or diapers and sitting.
Dr Rosemond has written many books and has a nationally syndicated newspaper column. You can also check out his website, although I forget the exact address. Good luck!