The reduplicative propositions about Sally and Bob do not boil down to contradictions. But ‘Christ as man is mortal and Christ as God is immortal’ does boil down to a contradiction. For Christ’s being a man includes his being mortal, and Christ’s being God includes his being immortal. Since Christ is both man and God, he is both mortal and immortal – which is a contradiction.
In sum, the reduplicative strategy is of no use in showing the logical tenability of the two natures doctrine. The schema is this:
S. x as F is H & x as G is not H.
(S) reduces to the contradiction x is H & x is not H if the following condition is met:
C. F-ness entails H-ness & G-ness entails non-H-ness.
The Sally and Bob examples fit the reduplicative schema (S) but do not satisfy the entailment condition (C). Thus they are DISANALOGOUS to the Christological cases which do satisfy (C).
The problem I see with this is this:
1) Being human does not entail being mortal; there is no reason to think that an immortal human is logically impossible. It is, of course, generally impossible; but the impossibility is an impossibility under normal conditions, or under most conditions, or something like that. So there is no entailment in that direction.
2) Being God does not entail being immortal simply speaking, but not being subjectible to death in the respect in which it is God. In most contexts we can simply drop the qualification, because it doesn't do any work in most contexts. Nonetheless, it is there, and I think this is the primary point of the reduplication strategy; being God doesn't entail anything about being human, even if what is God is also human, because what being God entails is entirely under the condition in the respect in which it is God. This will be true for all the properties attributed to God: invisibility, intangibility, immateriality, eternity, immutability, simplicity, etc.
3) If death is the severance of soul and body, or even the cessation of activity of the body, God is immortal simply because ordinarily God does not have a body [by which I mean it isn't entailed by God's nature that he have a body--ed.]. Thus, even here we can say that being God entails being immortal only under conditions in which we are considering God as not having any body at all. If there is a condition under which God can take a body, then God can die in virtue of having that body. In other words, divine immortality (in the respects in which God can be considered without a body) does not exclude divine mortality (in the respects in which God can be considered to have a body).
Reduplication, then, does avoid any logical problem. Other things I have said about reduplication (in other contexts):
Jottings on Reduplication
A Clarification on Reduplication in Christology
(The Maverick Philosopher has been discussing Trinitarian analogies, too; some great stuff there, all worth reading.)
