I have a pair of twins in my year 6 class and they are just fine together. They want to be in the same form class in secondary, though accept they may not have all lessons together. The mother is keen for them to stay together as long as they wish to and we can see no reason to keep them apart. They each have their own identity and set of friends. If you ask for the ed psych's recommendation then it would be a good idea to follow it, if it agrees with your professional judgement. However moving any child part way through the year will cause problems for the one you move. How will you decide which one stays and which one goes? What about continuity of learning so near to Sats? Are things so bad that it is worth the upheaval and distress for such a short time? Only your school can decide what is right for those children.