Ok, slightly confusing question, but I'll try to be as succient as possible. Need help for the best way to write this character.

In my next book, one of my protagonists (Kalak) gets possessed by a kind of ghul that takes him over (Mordak). This happened briefly at the end of my first book, but I expanded it a little more on this version.

What happens is that the character often speaks in two voice (one out loud, the other in his mind). Basically how I've written it is that when Kalak is in control the character is referred to as Kalak and Mordak speaks in his mind, in italics: Here's an example below.

You don’t sound too happy, the dark voice echoed inside Kalak’s mind. I would have thought you would be at least a little grateful.
‘Mordak, you son of a bitch!’ Kalak cried as he heard the vile voice of the ghul inside him speak.

However, there is one part where Mordak becomes the dominate personallity and then the roles are reversed. I'm trying to write the best way so that people won't get confused, but it's quite hard! Anyone got any advice. If I haven't explained it correctly, please let me know.

<Added>

Sorry for the werid symbols in between the letters. For some strange reason it did not come out with the apostrapies when I copied it into the post.

I have to admit I haven't actually read the book, but Stephanie Meyer's novel Host has talking souls inside human characters. Maybe it's a format that'll work for your character too?