Conjoined twins aren't using each other's bodies, they literally share the same body. As such, this becomes more of an issue of democratic decision-making, rather than individual consent. As an analogy, imagine the body is like a home; a pregnancy is like a guest that is staying in your home, whereas conjoined twins are like two roommates each sharing the same home. A guest in your home does not have any say over what you choose to do with your home, how you decorate it, etc., because it isn't their home, and they are only allowed to be in your home for as long as you choose…  Read more to allow them. On the other hand, two roommates who share ownership over the same home will have to make their decisions on what they do with the home, how to decorate it, etc. as a democratic decision, since they both own the space together. In the same sense, a fetus has no rights over what you choose to do with your own body, since only you can give consent over your own body, whereas conjoined twins each share the rights over the same body, thus their consent must be agreed upon by both. Without a mutual agreement, the consent cannot be valid, thus any such action against either twin would be a violation of consent.