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.