I'm exclusively a one-on-one role-player, and over the years I've learned that it's really important to find a partner that really jives with you first and foremost. Since there aren't any other players that can help move the role-play along, you have to find a comfortable balance just between the two of you. For me, personally, I find it very difficult to write with partners that are super passive and don't contribute much to the story on their own, I.E. not pushing the story forward, or waiting for you to introduce new characters or ideas. If my partner does this, then I lose interest very quickly.
It's a delicate balance, and it can definitely take a while to find someone that you work really well with. Once you do, though, it's easy to push a story along. Just make sure you keep a line of communication open, and make sure your expectations are made clear.