Except there's the inciting incident, which in your example is the accidental ripping of the two people's organs out and creating a person from them. The Medical staff all say it can't be reversed, and just as everyone comes to accept this new person they figure out "oh wait we can undo it"
You can't morally say it's acceptable to kill someone against their will to save one life, two lives or a hundred. No person should be able to be able to decide this. Had Tuvix volunteered for it that would be different, but he didn't. You can call him a coward if you want, but last I checked that's not a capital crime.