Most importantly, Tuvok and Neelix were dead by the time Tuvix came to life.
As I've said, I do not accept the validity of this statement at all. Consider that there is a very clear Trek-universe precedent for the situation in "Tuvix": namely, Trill joining. In that case, as with Tuvix, two personalities and two bodies are blended into a single composite entity. Would you claim that, for instance, Jadzia and the Dax symbiont both died when they were joined into Jadzia Dax? That's impossible, since there was at least one subsequent occasion where they were separated and Jadzia's original personality re-emerged; and of course the Dax symbiont lived on as a distinct entity after Jadzia's actual death. And we know that previous hosts' personalities live on in the symbiont and can "live again" through the zhian'tara
For that matter, when the Companion merged with Nancy Hedford and they became a single personality, that didn't kill Nancy; on the contrary, it saved her life. She continued to live on, as did the Companion, but they were a single blended personality. And what about the blended Spock/Kollos entity that existed for a few minutes during "Is There in Truth No Beauty?" It wasn't just Kollos taking over Spock's body, it was a blend of both their personalities. Was Spock "dead" for that period of time? Of course not.
So if Tuvok and Neelix were dead when they were combined into Tuvix, that would mean every Trill joining in history would've been a double murder. But Federation law clearly did not define it that way, since Starfleet doctors are known to have assisted or participated in Trill joinings (for instance, the emergency joining of the Dax symbiont and Ezri Tigan). Therefore, Tuvok and Neelix would not have been considered dead by Federation law -- simply joined.