I began writing scripts with two other people around 1940. Up until then I wrote alone, and found that I had no difficulties. But in writing alone there is a danger that your interpretation of another human being will suffer from one-sidedness. If you write with two other people about that human being, you get at least three different viewpoints on him, and you can discuss the points on which you disagree. Also, the director has a natural tendency to nudge the hero and the plot along into a pattern that is the easiest one for him to direct. By writing with about two other people, you can avoid this danger also.
I wish this were a more common, accepted practice.