Well, I do think there's a difference in that Tuvix was an accidental creation whereas the Borg's creation of Seven was a deliberate imposition upon the original personality, but I'd also point out that I do have some issues with how Janeway ignored Seven's request to exist in the manner she wished to (at the time). I suppose a case could be made that she wasn't mentally competent, though that raises a whole different set of issues. In any case, unlike Hugh, Seven did clearly state that she did not wish to be de-assimilated.
If killing a child would bring back two heroes on the scale of, say, Captain America, do we do it? If we don't do it, we're effectively killing those two Captain Americas? Rhetorical.
SG-1 covered these sorts of issues rather well in "Pretense", though in that case preserving the imposed personality (more or less) was an option.