Pediatrician's advice or not, twins are separate individuals. They have needs that cannot be generalized any more than you can generalize whether any other child would thrive in this situation or that.
My twins are 7, they've been in the same class (preschool) and have been in separate classes for kindergarten and first grade. They have done great in both situations.
They have made friends individually, and they have mutual friends. Most of the kids at their school know that they are twins and invite both even though they are in separate classes.
As their mother, you know most intimately what each child needs. I have always made the point to treat mine as individuals - We decided not to give them cutesy twinsy names, I dressed them differently, I make them each a cake for their birthday, I take turns taking one on errands with me - and it still doesn't diminish the special bond that they share. They still change beds in the middle of the night to sleep with each other.
Oh, and eventually, you will hear from one or both that the other 'bothers' them.