Like I said earlier, this looks like a cold way of approaching the situation, but this is why justice is depicted as wearing a blindfold. There is way too much emotion to go any other route. With no good answer, one can only choose the lawful one.
So what's the Federation law and how do the facts--as set down in the episode--pan out?
1. Tuvok and Neelix dematerialized in a transporter stream and rematerialized as one being who was a combination of the two at the molecular level. This being did not exist before that point so had no culpability in his own creation. He did not cause the accident. He was the result of the accident.
2. This being was a unique and new lifeform. Sentient lifeforms have rights under Federation law. Rights are not based on whether or not one finds the lifeform odd or creepy. Nor are they based on an officer's whim.
3. This unique individual called himself Tuvix.
4. He was a sentient being. He thought, he was self-aware. He was assigned to the bridge and proved his competence there. His thinking was not Tuvok at some times and Neelix at others, but a unique combination of the two. As an example, he used intuition and logic to solve a problem in hours that Tuvok had said would take weeks using logic alone.
5. Captain Janeway commended him in her logs. Her first officer described Tuvix as more than the sum of his parts. This is further evidence that he is a sentient, unique being.
6. The Hippocratic oath forbids the Doctor from performing a medical procedure on a patient who has declined it. Tuvix, as a sentient being, has the right under Federation law to decline. The Doctor acted as he should have under the dictates of his professional oath.
(An aside here... The only way current transplant law fits as a precedent for this case is in the cases of live donors. We cannot force a living donor to give up a portion of his or her body to save another--not even an identical twin--if the donor forbids it. No one can force you to risk your life to save another).
Under Federation law, as set down in episodes across all the series, and under his own professional ethics, the Doctor acted appropriately.
Contrary to assertions in this thread, it is not moral under *any* system of contemporary human morals to kill another to save a loved one from death. One might wish he could, but to act upon that wish would be murder. (On a personal note, I'm really glad the thought never crossed my mind, because I'd probably still be working with a shrink a decade later, wrestling with the knowledge that I could even conceive of such a thing...)
There are actually cases in contemporary medical law (not transplant law) that could be argued as precedent: under the law, one cannot force a woman to have a cesarean to save the life of her unborn child. Not even if she is dying.