A director is someone who directs the entire film or TV show just as the script demands and makes changes if needed. They handle the way the film is shot dramatically and artistically but they also manage other aspects of the film. A co director is someone who helps the director to direct the film in another perspective. He/ she are as good as the director and as also known as assistant directors of the film.
A film or TV show can have one or more than one co directors depending on what kind of genre it is. The more directors the more the films or TV show is interesting as many people have worked on it. Directors can be co-directors for other films or TV shows and vice versa.