If they were dead they can't be resurrected. That's why it's such a neat dilemma.
No, it's why the episode "Tuvix" can more easily be compared to a work of fantasy (wherein death may be reversible) than a contemporary moral dilemma (since death, as we understand it today, is currently irreversible).
Tuvok and Neelix are by any reasonable standard dead. Significant portions of mass that comprised their bodies -- including their brains -- are apparently lost after roughly half of each is combined into Tuvix. Their bodies have ceased to exist, and a new body been created. The unique mental patterns of each have ceased to exist; their minds have ceased to exist, and a new mind has been created. They are, in essence, Tuvix's deceased parents.
And, as was asserted in the episode itself, Tuvix is, by any reasonable definition, a separate and unique individual.
Ergo, the act of resurrecting Tuvok and Neelix inherently means killing Tuvix. The closest comparison would be to a fantasy story in which magically resurrecting two people requires killing a third.