[PHI 1000] Why won't you die like a nice undead?

On the NY Times' The Stone, Gary Gutting takes on the admirable task of defending the Knowledge Argument and the Zombie Argument. Gutting puts the Zombie Argument specifically in terms of logical, rather than metaphysical, possibility. For example, he writes:

The basic premise of the Zombie Argument is that my zombie-twin is logically possible (and therefore that laws of nature connecting physical make-up with experiences are not logically necessary, even if they are physically necessary). [original emphasis]

From the logical possibility of a zombie-twin, Gutting claims, it follows that

my experiences involve something beyond my physical makeup. For my zombie-twin shares my entire physical makeup, but does not share my experiences. This, however, means that physical science cannot express all the facts about my experiences.

But does it really follow? How does it follow from "it is logically possible that I have a zombie-twin with the same physical makeup as mine" that "my own experiences are not entirely physical"? After all, the first claim is about what is logically possible (namely, the idea of a zombie-twin is not logically contradictory), whereas the second claim is about what is actual (namely, the actual phenomenon of consciousness).

Consider the following:

1. It is logically possible that terrestrial organisms are silicon-based.
2. Therefore, terrestrial organisms are not entirely carbon-based.

Does (2) follow from (1)? What, if anything, can logical possibilities tell us about what is actual?