Tuvix is manipulative and desperate.
JANEWAY: Aren't there two other lives to consider here? What about Tuvok and Neelix? Two voices that we can't hear right now. As Captain, I must be their voice, and I believe they would want to live.
TUVIX: But they are living in a way, inside me.
TUVIX: Don't you think that I care about Tuvok and Neelix? Of course I do. Without them, I wouldn't exist. In a way, I think of them as my parents. I feel like I know them intimately.
So he tries to say they are still alive, inside him, and that should be good enough BUT he isn't willing to accept HE will be still alive in them once the transporter accident is rectified.