There is something known as ahybrid, where parts of two gentically distict individuals are combine to create a new. So if we create a new plant i.e a wheat planet that can survive in more arid regions. Is that not still a living organism?
So in "Tuvix" we have organism 1 Tuvok and organism 2 Neelix which are combined tocreate a new organsim. True in nature the orginals are generally not lost.
Now the closest case we've seen on ST that might come close is "Measure of a Man"
And on determing if Data was a life form or not we had three criteria
3.>Consciouness (aka a Soul)
Well Tuvix was definatly Intelligent, he seemed self-aware as for the 3rd point just as in the case with Data it can't be proven. But Data was at least granted the right to explore that possibility.
This isn't the same, Data is an individual on his own. Created yes, but two other people didn't have to give up their lives for him to exist.
Tuvix didn't cause the accident. He was the result of it. He meets the criteria to be an individual... the same ones Data did you admit... but just because he refused to sacrifice himself for the benefit of two others, you'd deny him his life? I just don't think in a moral society, any authority should be able to take an innocent life against their will regardless of the circumstances.