My identical twin daughters strongly prefer having their own teacher, classroom, and friends. Some friends overlap, but not too many. They need their own identities and really know what's best for them.
My district does not usually allow twins to be in the same class, however, I will be receiving twins this year because they will both have IEPs next year and I have the only inclusive first grade classroom in my school. The parents were very apprehensive about putting the kids together- apparently they fight a lot. They came in last year to observe my "structure" and then agreed to put them together. I'll have to keep the kids east and west next year.