TOW was not written with a co-author. Soundstorm was added long after (years I believe) the story was published. KF and SS were, from rumors, trying to convert the storygame to another medium until they quit. (It might have been an app or COG, I can't quite remember nor cared enough to remember.)
Achilles, on the other hand, was actually co-authored (unless you're on of those conspiracy theorists that believe Chase and Steve are the same person).