Good morning! I wanted to take a moment to share my views on the twins in school. I am a kindergarten teacher and I have been a party to both ways of doing it! I'll tell you the pros and cons of what I noticed.
Having both in the same class makes it easy on the parent, and in the beginning, easy on the twins themselves. They've already got someone in the class they know so their fear is lessened. BUT, because they already have somebody they know they are less likely to branch out and make new friends. They sit by each other at carpet and work time, they play with each other at choice time and recess, etc. Also, because in all twin situations one twin is more dominant, that twin tends to make all the decisions, AND tends to do the thinking for the other. I had fraternal twin brothers last year and one did all the work for the other, all year long. Even when I would separate them, the one would somehow find a way to end up telling the other how to do the work. Consequently one was beginning to read by the end of the year and the other still couldn't master their letters and sounds. This could've been just the way it was going to work anyhow but I tend to lean more towards the fact that the less dominant boy would wait until the other could/would do the thinking for him.
In that same twin situation those boys, now in first grade are struggling in different ways. They are separated this year and the less dominant boy is still struggling with his letters/sounds (because in first grade that is not the focus, so he is behind in that concept already) but the more dominant boy is struggling with some pretty serious behavior problems. Who knows if there is a relation between being separated and his behaviors- the less dominant one was always more of the "friend-getter" in my class!
Looking at the friend perspective, that was somewhat of a tough one on those boys. That actually caused a bit of a rift between them because they both wanted to be friends with one boy and he could only play with one of them at a time. The boys would actually fight over him.
In talking with Mom and Dad at conferences too they confided in me that doing homework was very tough on the boys. Because one was more advanced in his academic skills homework was a breeze. The other struggled. The more dominant twin would almost taunt at times that he was done already and got to go play. The parents would separate them for homework, and ended up doing homework on different nights with the different boys. And homework in our class was never tough!
On the flip-side having them separated from the get-go makes it a little more difficult on the parents as well as the twins in the beginning. But, because they are not together it forces them to branch out and make new friends and learn who they are "alone" instead of as a team. You know better than anyone how individual they are, but do they? You also know that no matter what, they are twins and they will always have a bond. Quite a few years ago I had identical twin girls that mom separated. The girl in my class took to it like nothing else, breezing in everyday, making new friends, having the time of her life. The other one though would cry everyday and not settle down until her sister came to see her. BUT, it turns out she had some developmental/emotional issues anyway. Those two have never been together in school and do beautifully. For mom it was hard because there would be two different homeworks, two different conferences, two different sets of expectations. Part of that was the other kindergarten teacher's and my fault- we weren't on the same page. That has since been remedied and most all grades in all schools tend to be more of a "team" and work together, having similar expectations and homework, etc.
As far as the concept of having mutual friends and hurt feelings over not being invited to birthday parties, etc., that's sort of a fact of life. Even in my own family with children 2-3 years apart there are hurt feelings when so and so gets to go to a party and the other doesn't. It's what kids need to learn they have their own personalities, own friends, own individualities. And if they have different friends it won't be so tough for one to realize they don't really know Johnny so why would they go to his party!
I guess in the long run I am saying to separate as soon as you can, although only you can determine if they are truly ready. You might want to start now sending them out on separate playdates, or sending one off with grandma for a weekend, and then the other a different weekend. Get them used to the idea of playing with others without the safety net of their sibling, making their own decisions without relying on someone else. And if you can already see a difference in the way they learn or if one is more academically advanced then the other, I completely suggest separating them.
I hope this all makes sense and that I didn't ramble too much. I wanted to give you as much ideas of what I've seen as I could!