The King book was The Dark Half... not one of my favourites.

Sounds like an interesting concept. I mean, when the boy side gets the boy haircut and the girl side complains she doesn't look "right"... you have set yourself a challenge that will be very interesting to see how it turns out. Good luck. You've got this. You have a concept, and you have an idea of what is going on, and I am guessing the conflict is going to be internal? Or will you have an external antagonist?

One thing I would recommend is finding some opposite gender twins to find out how - even though they don't share a body - they differ and are similar. Research with real people! Writers hate that!