The thought had originally crossed my mind, but I chose not to because the two systems are *so* similar that it would be duplicating the same content:
At the kernel level, not accounting for Kexts (which are naturally platform/hardware specific) the two systems are 90-95% identical - it's the same xnu, with a bunch of CONFIG_* conditional compiled code (which used to be leak for the longest while). AMFI/Sandbox are another two special kext differences, but the rest is largely the same.
At the user mode level, There are more differences. Conservatively, however, I'd say that the two systems are still 60% identical - Private and Public frameworks are different, and the filesystem structure (and obviously some subsystems which exist in iOS and don't in OS X).
The two volume split is thus for user/kernel mode - which enables me to discuss in both cases the common denominator, then split in relevant chapters or sections to the iOS/OS X idiosyncrasies.