Hi W....I have twin 7 month old girls! A few things to try would be separating them at night temporarily...we did this when we had to let one daughter cry it out at night b/c she was becoming inconsolible even in my arms at night. It took about 3 nights, but now she is a great sleeper...splept through the night completely for the second time last night. Once she got the hang of sleeping better and soothing herself, we moved her sister back in the room (We temporarily put her in my bedroom in a pack-n-play). You can also try letting her cry in the room with her sister. Usually my girls sleep through each others cries at night.
Also, if you don't have one, I'd suggest a support group for twins. I'm a memember of www.twinstuff.com and love it. The forums are great support and I'm sure you'd find lots of other advice from mom's that have been in your same shoes!!
I like the book Healthy Sleep Habits Happy Child by Weisbluth...and I know he has a book out geared towards twins now too, although I haven't read it.
Good luck - here's hoping you get that much needed sleep soon :)