Tell me about it ;) thats why you don't see a lot of great dresses on the market, there is no way for a long dress to conform to both legs all the way down and stay in one piece. the general solution is to make the skirt one or two bodyparts with no leg seperation that bend either on their own or with one leg, and a lot of morphs to make it do anything more complex generally requiring people to pose the dress than pose the legs to fit the dress.