My friend has boy/girl twins, born premature, both immature today. The boy is an average student and the girl was held back in second grade, but still has troubles. The kids are very, very small for their age, and with that and the immaturity, if it were me, I might have kept the boy back too.
but in general I don't think it's fair for one of them to repeat a grade if not necessary. And if they're both advanced, one probably won't like the daily comparisons with her twin if they're in the same classes and she gets C's when her sister gets A's. Geez. I don't envy people who have to decide this...