I have B/G twins and had the dilemma of when to have them start kindergarten. They were June 2nd birthdays and my daughter was ready for kindergarten, but my son was not. Having been a 1st grade teacher, I could tell which boys had waited to start K. Older boys always did better and were more confident. My daughter is very tall (off the charts) and I am tall and had been teased into my teens. If I held her back, she would have looked like a freak.
I did not consider starting them different years as an option. I had heard from 2 sets of adult twins that this had been the case for them and it was a mistake. It was bad for their relationship and their esteem. If my daughter had not been so tall, I would have had her wait a year in a heart beat.
Family and friends who are not twins, did not see the problem in separating my kids and accused me of not treating them as individuals and doing what was best for their individual needs. Yes, they are individuals, but they are also twins and that is a big part of who they are. That relationship deserves respect and it impacts their sense of self.
Fortunately, we were able to solve our problem by putting them in a private school that had mixed age classrooms. So when they were 5, they were with kids who were late 4s, 5s, and young 6s. They will be in these mixed age groups (now 2 grades per class) until jr. high. Then we will have to address the issue again.
These are very difficult choices for parents to make. I hope yours works out the best for your kids.