I always give at least 2-4 hours of time for newborn shoots because they really don't cooperate the way they should! It takes me 3 hours to get them to stop crying, being hungry and finally asleep to spend 5 minutes actually taking pics!
The twin shoot I had was quite a treat, try getting two of them to behave at once!
It helps to put a heating pad or blanket underneath whatever blanket you have them on, it keeps them comforted and cozy.
Also having mom touching them or patting their butt while you position them keeps them from waking up.