But it's not the same thing. It's not sacrificing one life to save two. The problem these analogies completely miss is that there were two unwilling participants in the situation. The situation is like nothing that has ever happened, so you can't compare it to ending one life to save two in a terrorist attack, or using a baby to save two parents. Because In these cases everyone involved in the terrorist attack and the parents and the baby all have a right to their own lives. Tuvox's life wasn't his own, it belonged to Tuvok and Neelix. In a sense, he existed without their consent. A flower first made the choice for Tuvix to exist, then Tuvix tried to make it again. Problem is, it wasn't the flower's or Tuvix' choice to make. It was Tuvok and Neelix's choice, and the captain rightly acted in their best interest.
Tuvok and Neelix and a plant were turned into one being that had no right to exist. He had as much right to exist at the expense of Neelix and Tuvok as the captain had the right to separate them again. There is no malice in correcting a accident that never should have happened in the first place.
DS9 had a similar dilemma when they got stranded on that planet, and built a society there. They ran into all their descendants who told them when they tried to escape, there would be an accident and they would be flung X years into the past, and be stranded there on the planet forever. But wait... maybe Captain Sisko figures a way NOT to get flung back. What does he do? I think he chooses to let the accident happen. But there's a way out! Odo of the future is the bad guy and the whole thing never happened. Typical self-serving Odo.
They should have had Tuvix get seriously Ill, necessitating the split. Then everyone's conscience would be eased. Maybe that's how the 60's show would have gone.
But screw it. If I were Sisko, I would have got the hell out, and If I were Janeway I would have saved my friends and officers. There is no malice in saving yourself, and/or correcting a science fiction accident.