First, let me say: great idea! The contrast you made is awesome; Unohana with white background and covered neck (in black hair) and Kenpachi-Unohana with an exposed neck and a black background - it's like she stepped from the darkness into the light. It clearly shows the difference between two personalities. Further more, the details (blood on her face, loose hair, unsheated zanpakuto) makes the change even more obvious; but at the same time, the frowned eyebrow gives her a touch of Unohana ''the healer''. I see a lot of attempts to show the dark side of a character, however they often make him/her all monster-like and the connection between an original and 'the dark side' is mostly lost, but you kept the link between them with a simple line.
However, I found the shading in the Kenpachi-Unohana's neck a little off. I'm an amateur so I can't say if it's the density of the lines or that little curve on the lower part of the neck, but on the first glance I thought she's wearing a scarf.
Another thing about the highlighted areas on her braid - it looks like there is something entangled in her hair. Maybe instead of using two set of lines it would be better to use just one set of lines like you did on the Kenpachi-Unohana's hair.
I truly love the choice of word you used to describe them - pure perfection.