A siamese twin would not be morally justified on the basis of bodily sovereignty to have his twin killed and removed. Even those his twin is attached to him in such a way that they cannot be removed, that does not warrant the killing of the twin, because he has a right to life.
No. A Siamese twin has no right to kill off the other twin. Removal should not be allowed unless the other twin is no longer alive and is simply extra skin and bones for the living twin. Siamese twins are extremely rare, and so is the nature of having one removed, however I would hope that doctors would follow a code of ethics with regards to those types of decisions.