Not my idea either. My point is simply that by giving this example of something we've actually seen in the show, once you start fiddling with transporters, and getting accidents like merging two persons into one, and splitting one person into two, there is no longer an easy frame of reference of which is more 'real', or who of them has more right to live. Except maybe for the 'status quo' argument. (which could be perfectly valid, can't really judge that).
Perhaps they should just as bluntly have said: we think Tuvok and Neelix have more right to live, as they were naturally born that way, were here before Tuvix so they have older rights, and we've known them for a longer time.
It wouldn't have sounded like a sophisticated ethical argument, but it would probably have been the simple truth.