[@Dusksong]

Collaborative writing and solo writing still deal out the same number of points per post (unless you're part of the Helping Hands Guild or something like that). 

[b]EDIT:[/b] No, if someone allows you to write as their character for the sake of advancing your story line, the character that [b]does not[/b] belong to you [b]will not[/b] receive any trainer points.

You're own characters can interact with each other, definitely. They just won't receive any bonus points. That would be broken.