To make it fair we both set up a character and name the greatest fears that character has. We both play the King of Nightmares for the other to keep it interesting. So the deviding roles at a minimum would be:

Victim: You
I'll play the Nightmare King then and your guide(s)/protector(s) and it will work the same the other way round.
Victim: Me 
You'll play the Nightmare King and my guide(s)/protector(s) while working with the top 3 fears we've listed for our characters.

Seeing as the dreamworld is a mix of thoughts you control and can't control it leaves us both with the opportunity to add bits and pieces to the world.

Hopefully I explained it clearly enough.