As the mother of twins, although mine are boy-girl twins, I can well understand your concern. Also relevant is whether your girls are identical or fraternal, as it always seems more of an issue if the twins are identical.
Your decision should be based as much as possible on the relationship between the girls (though some school districts have policies that take the decision out of the parents’ hands). If the girls are extremely close, get upset if separated and can’t function well without one another, then by all means urge the school to put them in the same classroom, at least for the first year or two. If they are reasonably self-reliant and able to form friendships with other children on an individual basis, I would recommend separating them.
There is no question that twins need some life experiences without having to have everything filtered through their “other half” if they are to develop their own self-concepts and personalities. And even identical twins have some differing personality characteristics.
I don’t know about yours, but sometimes one twin is a little slower than the other. Often there is a big difference in the birth weight of the two babies, and the smaller one needs extra time to catch up. If that is the case with your girls, then separation is even more important. The faster one needs the opportunity to move ahead at her rate and not feel that she must stay behind until her sister has reached a certain level. And the one who needs more time can benefit from not always having to compare herself with her twin.
As is obvious from my answer, I favor separation. The only “con” would be if acting independently seriously upset and inhibited one or both of the girls. This does not usually happen, and both can thrive by being allowed to be a bit more on their own.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.