While commonly thought, this isn't how twin usually studies work. The way it works is you look at the correlation between identical and non-identical twins on some given thing. If there's a much stronger correlation between the identical twins, then it's probably primarily genetic. What this does is helps to eliminate environmental factors because identical vs nonidentical twins will both be raised in basically the exact same environment.

So take height. If identical twins have identical heights while non-identical twins have varying heights, then it's safe to assume height is largely genetic. Interestingly separated twins would actually be worse in many cases because you reintroduce environmental deviation. For instance with height, differences may well be down to e.g. nutrition, but when you have them in the same household you can usually assume roughly identical nutrition.