I find the GM x player mix just never works out and you're always left carrying the story. Most of the RPs I've had that have failed are because people expect you to create ALL the details when the best stories are the ones where it's a balance between your combined interest. 

I also like to add subtle hints for when the other character is near, or able to interact with my character. It's a real pain when the partner creates what feels like their own solo adventure, and it's also frustrating when they fill it with a spectacle of NPCs (one or two no name fillers are ok). I've backed out of 1x1s for both these reasons.