Hi E. -
I have been on the other end of this decsion and from a teachers point of view, I would seperate them unless the school wants them together for some reason. I taught 2nd grade for several years and had 1 set of twins that the parents insisted be kept together (had been from K) and I had discipline problems with them. We also had a set of twins another year that the parents had seperated for the first time and it was a nightmare. Both of them regressed terribly. Both were non-verbal for quite a while, and the one I had went back to sucking her thumb non-stop and crawling instead of walking around the room. It was months before she would actually speak to us and her classmates and not suck her thumb constantly.
I know you want to keep them together, it seems like the natural thing to do, but I would really consider the 'trama' it could cause later when you do have to seperate them. Plus it's good for them to get to know other kids and have seperate friends. They are, after all, 2 seperate children who will lead seperate lives. They will always be sisters and have that special bond that twins have, but they also need to be individuals and be allowed to develop as individuals.
Good luck in which ever you decide to do. :)
J.