I am a mother of 2 yr. old twins (boy & girl), our first & only children. My husband & I had the problem of BOTH kids waking up at 5:30 am & whaling! We ended up giving them a bottle until they reached a year old. This was the only bottle they had in their beds. Ours were preemies & we just got use to feeding them every 3 hrs. & didn't stop! They got huge. Wish I'd been a mother first before these 2 b/c I would have probably done things differently (weined night time bottles sooner).
Like you, we couldn't have one child waking up the other child! We moved one kids bed into the guest bedroom to help with the whaling. Are your twins in the same room? They seemed to sleep better separately.
Back to the little twin...........I say, "Feed her" & she will probably sleep longer. Since she did this 2 mos. ago, she's probably just in a growth spirt. Make sure she's very full on cereal (or something that will stick to her ribs) prior to bed also.
I understand the morning wake time. The sleep deprivation with twins is like nothing I've ever experienced before. YOU ALL NEED YOUR SLEEP!
AS far as naps & not being able to stay up 4 hrs. from 3pm - 7pm, give the last nap at 5 or so. You may need to move bedtime out some. With us, my husband didn't get home until 6 or so & he wanted my kids to have a later bed time, 8-9ish. I don't know your situation but maybe a later nap & later bedtime would be better if they are more tolerable??
You know 5-7pm is "bewitching hour" for all kids. I found it was the worst time of the day. As a family, we started going outside during this time (almost religiously, everyday). One so we could all get some fresh air. The walls started caving in at 5pm! Also, this gives the kids that last bit of sunlight to take in & when it becomes night, they sleep better.
Last, we covered our son's window in his room with stuff to make his room darker. He's still the first one up in the house, 6:30. Now at 2, we just leave him in there until we get up around 7:15. He sweetly sings, plays & doesn't mind just being awake.
Best of luck. You're not in this alone. It seemed like 9 mos. & then 15 mos. old things changed & got better.