What a predicament . . . I feel for you. I'm also a grandma, but all 3 live with their parents : ) -- 170 miles away : (
I think this is one of those 'simple, but not easy' situations. I'd give a matter-of-fact explanation to BOTH of them that, although they'll always be super-close, life will present many circumstances that they can't be together. Be sure to 'accentuate the positive', though, that this lets them each build upon their separate personalities with lots of other cool kids/folks/teachers/people and they'll have so much more to talk about and share with each other when they DO get together!
Since the school has already placed them, I don't think I'd have them reassign their classes, but I definitely WOULD speak with BOTH teachers and come to an agreement of some sort as to what to do if one twin TRULY needs the other. They'll have to learn SOMETIME to be content while apart, though (and probably the sooner the better since separation will only bcome MORE difficult if they keep growing more and more dependent upon each other as a 'counterpart'.
P.S. After reading responses from other twins, I may digress a bit about not having them put back together.
It really does depend on several things, though. I was just thinking about it from the angle that if you have the school system to 'move' one of them this year, they might think that they can get their classes changed every year (kind of like with pre-school, which isn't required, but if a kid starts and then doesn't like it and the parents let him quit, he thinks he can quit again the next year when Kindergarten IS required).
Also, if this is causing YOU more grief than it's worth and you don't feel confident that you can 'make the best of the situation' and stay postive about it (I mean TRULY positive and not just trying to 'sound' or 'act' positive for the girls' sake), then by all means ask for different arrangements at the school! Their (and your) peace of mind really DOES matter!