Tuvix was alive for some time in which time Tuvok and Neelix could and maybe should have been considered legally dead. Tuvix wasn't killed to save 2 lives, he was killed to bring 2 people back from the dead.
If they were considered dead, then they never could have been brought back in the first place. The show starts off with the EMH saying that he can find a way to separate them to bring them back. The chief medical officer didn't consider or pronounce them dead, then there is no reason we should.