D.L.
I am a twin and have twins. My sister and I were together in kindergarten and separated in first grade. I put my daughters in the same kindergarten class. The teacher suggested I separate them in first grade because one was dependent on the other. When I told them they were going to be separated, they cheered. So even though they were just six, they knew they wanted to be apart. They are now 13 and very close friends. I did ask for them to be on the same "team" in middle school, because trying to keep track of their tests, assignments got confusing. They don't have classes together. I thought if they have the same team of teachers, their grades would be more comparable. We found in the past that different teachers grade differently.
I would ask the twins what they want to do and go with that. Hopefully they both want the same thing.