Well,
I am no expert on the twin connection. They have a very strong tie. They are also individuals. I assume that if they were not twins you would have each child in the class that is appropriate for them./
My son was held in preschool an extra year because he JUST MISSED the cut off date. He's very smart and ahead of the class in many ways EXCEPT for socially. He's shy and becomes a loner at times.
Being held back will be really good for his confidence level for the rest of his years. Also, I am planning on supplimenting his education with science and animals...extracurricular things he is "into". Each child has a special purpose. Different strengths and different weaknesses. I would focus on what the twin with the speech and circle time issues is doing really well. I would emphasise that a lot and try to find a way to tie it in with staying in preschool another year. I don't know what that would be, but you turn a negative into a positive. Life does that for us all the time. For instance, I told my son "honey, the reason you have seizures is because you have SO MUCH POWER in your brain. You are meant to become something great. Your head can barely contain all that power! I wonder what you'll turn out to be!"
He loves it. He had seizures, he's 90% better now through natural remedy....
Anyway, I think you have to treat the children as individuals and be patient with the twin bond. Try to engage them to help each other.....explain to them that every person has a different path. Who knows? Maybe the reason that the one who stays behind is that someone wonderful comes into your lives. There's no telling. I wouldn't feel bad about keeping my child back even if his twin were to move forward. They have a whole life ahead, they can't live at the same pace all the time. Also, I wouldn't want to keep a child who needed to progress from doing so just because their sibling wouldn't be able to do the same.
Good luck. Twins are so awesome! Good job.