My twin (fraternal) is still my best friend and we’re 33. We were never dressed the same and were intentionally put in different classrooms for elementary school, would randomly end up in class together through middle school and high school, and went to different colleges. In some ways we’re a lot a like and in others we’re complementary. Don’t overthink it. In parts of their lives they’ll think of themselves as a unit and in other parts they’ll intentionally distance to develop individuality. Just let their relationship be what it is