All siblings have to share, and its good to know how, and I never felt that I lost out on sharing attention. If anything I needed it less because I had my sister to give me attention (and vice versa). A sibling the same age really understands what you're going through, while a parent can only try.
Only time I felt a bit p**d off was when we were given a birthday card to share by friends and relatives. A shared present I understood, but getting your own card seemed a basic requirement to my sister and I, especially as we got older.
The other thing I would say, which is linked, is remember that twins are two seperate people, not 2 halves of the same, and should be treated as such.
And watch out for the homework sharing when they are older - my sister and I have a complete set of O levels, but only half each. We always split the work down the middle and it really caught us out when we got to the exams!
We decided at 18 that we needed to make the break and become full individuals in our own right, so deliberately went to different colleges at other ends of the country. I cried my eyes out the day I left her, but it was the right decison.
20 years later we live a long way apart but speak and email regularly, and would do anything for each other. Can't imagine life without her.
Your twins have an inbuilt support system, and will never be alone. what more could any girl want? Be glad for them (especially when they are both crying at once and you feel you can't manage . . )