I don't have twins myself, but a few friends of mine do. I live in Germany and I'm French, just to give you a cultural context. My observation and my experience talking to parents of twins is that there is no "good recipe" when it come to keeping twins together or separating them. There are many different dynamics in twin brothers and sisters. What I have gathered is:
- If the twins are really close to one another and somewhat dependant on each other, in a balanced way, you probably don't want to have them separated, because you don't want to break that balance and you want to give them as much comfort as possible. Being together for a while longer might be beneficial to their development.
- If on the contrary, the balance is that one or the other is completely dependant on the other one, and that one "rules" the life of the other, separating them might be a good way to restore independance and make sure one of the twins does not dictate the life of the other. Breaking them apart can be a beneficial thing as it allows them to grow independantly, have their own set of friends and experiences.
- If they form a block, ie. are very close to one another, but at the expense of other kids (they won't mingle with other kids because they have each other), you might want to separate them in order to socialize better, but then they might have a hard time on their own. This will depend on kids.
- If both are very independant to start with, you can choose to separate them or to leave them together, it probably does not matter, they'll do well either way.
In the end, your friend is the best judge of what her twins interact with each other and with other people, and I think she probably can go with her guts of what she thinks is best for them both. She should feel comfortable asking the kindergarten to keep them together or separate them, depending on what she has observed so far in the way they act between themselves and with other kids. If she's not sure, she can maybe tell the peole at the kindergarten to try one thing or the other and observe how that plays out, and maybe change groups along the way if she realizes that separating them was a mistake. We all make mistakes, the important thing is to observe and learn and be able to change course.