I can reflect on my own personal experience. I am a twin (fraternal, we look nothing alike) and love my sister to pieces,however, being in the same grade and sometimes the same class sucked! You cannot escape from each other. You are together at home, school etc and you are always being compared to each other. Even if you don't compare your children other adults and peers will. Which one is smarter, prettier , more fun to be around etc... Personally, I think it will put both girls in a box they don't necessarily want to be in and regardless of how close they are will put stress on their relationship and create resentment that can last for years. Just a thought even if your older daughter does not tell you that it bothers her, it still may.
P