My twin boys have been in the same class for 3 years, preschool thru 1st grade. For pre-school there was no choice anyway as there was only 1 full-time and 1 part-time class. When they registered for 1st grade my husband had orders for Iraq deployment and I requested they stay together another year as splitting them up and missing their father for 18 months would be a tough strain on them. Our problem was with kindergarten. I did fight for what I thought was best for my kids.
The pre-school teachers recommended spliting them for kindergarten so we went with it. At that age (4) we had one child who was pretty independent without his twin and one who always wanted his brother around yet is rather bossy and always nosy about what his brother is doing. Actually at age 7 we have the same thing. LOL We thought it would be good to split them up thinking the one would do better without his brother constantly on him and that eventually the other would get over it. In fact we had the complete opposite reaction.
Bossy nosy brother loved his teacher and fell right in with his own class not even wondering what or where his brother was. Independent brother started out totally missing his brother. They had different lessons, different homework each day. To add to it he did not like his teacher at all and felt she didn't like him. To make it a trifecta, he is also ADHD with major complusion control problems and was labled "bad" right off the bat by this teacher, hence always yelled at, in time out, even had a boy at his table realize he could blame my son for stuff and get away with it while mine got punished. 2 years later he still dislikes this boy from the beginning of kindergarten. LOL. We let it go about 3 weeks into school and then we had an incident....... I requested a meeting about moving him out of the class and got the song and dance about how they'd have to discuss it with the teacher and how it's not a good idea...basically got blown off without being allowed to talk about what happened. I held them out of school the next day and demanded a meeting. There I requested he be moved to the other class with his brother (there are only 2 classes per grade) or I'd put them both in a new school. We have our boys in Catholic school so there are 10 options within a 10 minute drive of us. I even toured one of the schools that morning with the boys. After I finally had a chance to talk about the "incident" the principal, who began out quite nasty with me talking about how difficult it was to move them, checking with everyone but the pope, etc... suddenly bent over backwards if I wanted him moved today or would we just like to take the rest of the day off and start over in the new room tomorrow.
Now they are going into 2nd grade and I haven't made a request either way. We decided to just take whatever room assignments they get. If they are together fine, if not then they are comfortable with school and friends and ready for time apart. I'd say it depends on the twins. Some twins need to find their independence away from each other and others can be their own person with their twin right there. My boys are the latter. Even in the same class they don't interact any more than any other two kids. They have their own friends, though being they are very similar boys in personality they like the same things and have some of the same friends. They are also frat twins so people don't confuse them for each other. Their friends don't even realize they are twins. (While they still don't quite get you can have a brother that ISN'T your twin LOL) I think that helps as well in that people are not seeing them as 1/2 of a set. Our school started compartmentalizing subjects so both classes get both grade teachers. One does the language classes, reading, spelling, english, etc.. the other does the math, science, health classes. So even split into different rooms they'd have the same subjects and same homework on the same days. That makes my life easier.
What I have recommended to other multiple parents I know is request what you think is right for your kids but ALWAYS state you reserve the right to change your mind should you feel that way isn't working and you want them back together/split up. I think the kids let you know when they need some space from a sibling....even a twin/triplet one.