Congrats on having twins! I can imagine your nervousness, and also your joy about having new babies around. Two thing I did that helped calm me down during my pregnancy. First I looked into meditation. It isn't crazy hard to do, and it isn't as "new age" as it sounds. All it is, is just relaxing your body for a while. If nothing else, it will help you to concentrate on the positive things for a short period of time. I was at first reluctant to try it, but was so glad I did after I got going. Just search under meditation, and there are lots of sites that can help you learn.
The other thing I would do is look into fetal monitors. That way you can hear your babys' heartbeats whenever you want to. This helped me for the first few months and calmed me down a bit when I was nervous. I didn't use it that often, but just when I needed some reassurance.
Hope that helps!
-Amy
http://www.parentingzen.com