As a twin mother I'm on the fence with this one.
My twins were split from year 1. I was fine about this, but mid year, one was under the care of CAMHS due to another boy in class picking on him relentlessly. It was awful. I wanted him to be back with his twin for support.
I was called to a meeting with the HT. When I got there she'd invited several teachers and the pastoral lead, who told me that it would be a mistake to put them together and that I needed to trust their professional judgement.
But this was heart and head. As their mother, I felt I knew best. I listened, but disliked the approach - there was just me and a room of teachers and the intention was to overwhelm me with 'one voice'.
I did keep them separated ad it was fine. They both flourished, as I still believe they would have together.
They were both treated as individuals and learned independence. It was good for THEM. As a parent it may well have been best for ME to keep them together
However, the dynamics of twins are complex and every set up is different. Schools often work hard to keep friends together in school, yet separating twins is often a given.
I think schools should consult with parents if a parent requests it and the decision should rest with them as it isn't always best to split and it isn't always best to keep them together either.