When my DW and I first started writing computer books, we would edit each other's chapters. This helped immensely in creating a cohesive style, which was reinforced when we had a good developmental or copy editor. (About 1/2 the time. The other half isn't something I can comment on in a family friendly forum, except to say that editors should stick to editing and not try to pretend they're technical writers. And that the writer's favourite and best word is "stet". Or when it's particularly egregious,
STET! 
)
One of the things that we know Lee&Miller do is to read and edit each other's work. Sharon has commented that they talk out differences of opinion, and try to reach a consensus, but ultimately the lead writer is the winner if it can't be resolved. And you're absolutely right -- they do an excellent job of having a homogeneous style or "voice". Readers have often claimed they can tell who wrote a particular section, but have just as often proven to be wrong. I gave up trying.