It is pretty late now, because people have all been told. Most schools will not be keen to move a child at this point unless it can be done without moving someone else to balance numbers, and even then are often not keen, because as others have said, it opens the floodgates and most people can think of a reason for their child to be moved. I totally understand that on this, schools often will be really rigid. Remember, it doesn't just affect your child but others too and if you are allowed to make a request, why shouldn't everyone else be allowed to as well?
That said, it can do no real harm to send an email with your request in it. Be polite and not demanding and you never know what might happen.
And for future reference, if you ever need to make such requests, it's always best to make them well in advance of groups being formed or lists published - although schools still might not take into account your thoughts, at a much earlier point, they have the possibility to factor your thoughts into their planning.....but once it's all out there are published and might mean others being moved to, it really does become pretty impossible.
And yes to twins benefitting from not being together. Don't spend much time on the issue with the kids and it won't be an issue for them.