It is difficult to generalise, because ecah case of conjoined twins is different. But most cases die before birth. Of those who are brn alive, most die soon after. Of those who undergo an attempted separation, most suffer the deaths of both, many suffer the death of one and the survival of one, and only a few result in the successful separation and survival of both. There are many cases where no separation is even remotely possibel and is never attempted (e.g. the Schappell twins).