I can't answer this question in regards to film, but I can for music, which is a similar creative process.
In the seven albums I've recorded, two of them were with a duo. While the albums came out nicely, I felt very constricted by my creative partner. Not because he was doing anything wrong, but I felt that when I had a good idea, it was left up to whether or not he also agreed. In my creative process, especially in music, and in our case, writing a script, I don't like to be constrained by a partner. I like to go nuts, and go off on a creative tangent.
As the people above have mentioned, I prefer to go off on my own at first, and then receive ideas and feedback about how to proceed and what edits should be made.