I rely on inductance to choose...
A 16k humbucker with an inductance in the 8H range ends with 2H of inductance in parallel and its resonant peak shifts up accordingly. Split, it would measure 4H and would be noisy.
A 8k humbucker with an inductance in the 4H range would sound too thin in parallel because of an inductance of 1H. But split, it measures 2H, which is the value of a Fender CS69, and sounds accordingly.
The main consequence of such factors is that a 16k/8H in parallel and a 8k/4H split have similar inductances, which puts their resonance at similar frequencies (not identical because of parasitic capacitance of their coils but "close enough"). So they sound close to each other... ;-)