My DD has fraternal twins in her gifted class but they are different genders. By all accounts, they treat each other as if they don't exist. I think that this would be much more difficult with two who are the same gender. In my DD's class there are only 9 girls and they have all been together way too long (5 years now with the same kids in the same gt class). They feud and bicker like siblings. I think that it would be hard to have a true sibling in the mix as well.
Whether it happens this year or next year, it sounds like your kids will be grouped together. I would approach the teacher/school admin with my concerns before the beginning of the school year. Do you have any research that you could share about how to handle twins in the same class?
As for the mixing gifted and struggling learners in the same class, I agree with the others -- ask for the data.
