If they're so close they feel like twins, and they both feel okay about the proposed situation, I'd go for it. If they're going to disagree and fight it'll always be something if not the skip, so I wouldn't worry about problems between the two of them. If they're a girl and a boy that helps too -- they'll be less likely to end up competing against each other socially. Just make sure they know that if one is skipped and one isn't, that doesn't mean anything, and even though they are siblings, they are people independently of the other and should support one another instead of being jealous of friends/grades/attention/the last cookie/etc.
