You are correct in being concerned about the emotional and social implications of splitting up your twins. You also need to be concerned, however, with placing your younger son in a situation, which will continually frustrate him because he will be forever trying to "catch up" to the others in his class (and his brother). My guess is that if you asked him whether or not he would feel more comfortable repeating second grade that he would say no. But he might say yes, if he is reassured that all else in his life that provides him appreciation and security will remain the same. He just might need the pressure taken off at this time for things that he is not quite ready to master developmentally. Kids can be made to feel stupid, disappointing, and "less than" by not developing (physically, academically, and socially) at the "normal" pace.
I would not allow myself to be greatly influenced by what other parents of other twins have done. Make a list of all the possible positives and negatives of both advancement and retention. Consult with a child therapist who may be able to offer some significant input after a few consultations with your son. You know your son better than anyone. What decision would be healthiest for him in both the short term and the long term?
I would also recommend a superb parenting book -- The Joy of Twins, by Pamela Patrick Novotny.