I wrote quite a long and involved story using both of my characters on EC who are brother and sister. It was done purely in story-style roleplays, without subjecting other players to letters written between the two characters. So I would write part of the story from the point of view of one character, and then switch to the other character to write a section of the story that was happening out of sight, or behind the back, of the first character.
It didn't exactly make the two characters enemies, but the sister was manipulating events behind the brother's back in a way he still doesn't know about. I couldn't say whether other players liked the roleplays involving both of the characters, but a few other characters have made reference to some events in those roleplays so I suppose they must have enjoyed reading them.
