I dont have twins, but my husband is a twin and his mom had to fight to keep them together since the schools think they need to be split up. The twins nowadays will always advocate for keeping kids together. Especially in your case, when they are already doing great, why mess that up?
My husband's cousin has a set of boy/boy twins and she allowed the school to let her split the boys up. Now they have a ton of problems, trouble concentrating in school, etc, that they didnt have before they were split up.
My suggestion is always to keep the kids together. Eventually, due to the way classes are in middle/high school, they will split up, but there's no reason to force it unless they ask for it.