I would be livid if a school had a "policy" of always splitting twins, and would, in fact, withdraw my DTDs from any school that brought such a policy in. Twins, like any other children, are individuals, and schools should not have policies for dealing with all sets of twins in the same way.
My DTDs are ID, and it would have been inconceivable to me AND to them to be split when they started reception. They are in no way dependent on each other, and on parents' evening, their teacher said that they do separate things and don't tend to stick with each other, but are always aware of where the other is. I have a friend with twins in the same school, and they have been split, because her DS, whilst at nursery, had become very dependent on his sister, and wouldn't do anything for himself. However, in both cases, it was up to us as parents. I spoke to the headteacher, and he made it very clear that their "policy" was to do what the parents wanted them to do. TBH, I find it odd that any school would have a rigid "we split twins" policy.
There is plenty of research to suggest that splitting twins, especially against their will and at a very early age can be very traumatic, and there is absolutely no evidence that it benefits anyone other than a teacher who might find it tricky to tell ID twins apart.