I have 4 year old identical twin boys. Even with identicals, we have to remember that we have two separate children. They won't always do everything the same way at the same time. With sleep, this is more than a little disconcerting. This is what I did to help my boys sleep through the night.
I put them on a pretty strict routine of eat, play, sleep (& repeat). When one would wake up to eat, I would wake the other to eat. They were on the same schedule. My boys were sleeping 5-6 hours a night by 5 1/2 months and 10-12 hours a night by 7-8 months.
Are your babies in the same crib? We started my boys out in the same crib and then moved them to separate cribs at about 5 months. They were smacking & kicking each other and waking brother up!
If you have your daytime & nighttime routine down, I would say that this is a (hopefully temporary) phase that you will need to help your daughter through. Hope this helps!