Hey Danny,
First of all, it's helpful to use a basic songwriter split sheet to establish in writing what your co-writer credits/splits are. It sounds like you've already addressed this but I would recommend getting them to sign something as well to clear your path.
When you have them sign off, include some language that mentions that they waive the right to refuse any license offered for the song. I'm not a lawyer and would recommend that you talk to someone professionally certified for the exact wording. This could save you some sweat down the road!
Again, lawyers are your friends
Good luck placing your tunes!
Dan
