Rand Al'Thor from The Wheel of Time is the best example of split personality disorder that comes to mind.
The only problem here is that it is a 14-book story and his split personality starts showing around book 5 or 6 and lasts for around 5 more books so I'm not sure you'd want to read this...
A piece of advice that I heard a while ago and had worked very well for me.
Write an introduction in those side characters' POV. Let them describe their history, motivations, personality, you can even let them express how they felt about certain major events in the novel.
Do this for your most...