I believe that this question can be tricky! However, if I were the parent of twins, I would lean toward separation. My reasoning would be so the teachers would not make comparisons (even unintentionally), reduce any competition, give each child an opportunity to blossom on their own merits and make friends independent of each other.
However, if there are limited options in the school and / or they both need special services where only one class offered them, then together would be a consideration.
It probably depends upon the children, but I think it is most common to separate them. Being in separate classes allows them to really be appreciated as the individuals they are, and to develop other close friendships.
My twins just started kindergarten and I have kept them together this year because of one twins anxiety issues. Each set of multiples are different. I know some twin girls that are separated in preschool. What are their personalities? How did they do in kindergarten? Were they together? Have they always been together? Do they sleep in the same room? Do you belong to a local Mothers of twins club?