I'm told co-directorships are not allowed by the DGA, which is silly, but as this may go towards the long-term mental health of directors perhaps this saves them on insurance payouts.
...Co-directing is somewhat insane when you consider the godawful number of decisions a film director has to make which now have to be filtered through two people. The two people contradict each other. They debate while the rest of the crew has to wait for instruction. They give everyone, especially each other, massive headaches. No matter how well planned, they will have sudden creative inspirations....often, against all that is holy, SIMULTANEOUSLY...which will send the ship flying in multiple directions.
...That said; it's good to go in with a partner.
...On an indie film set particularly, the director often has to be several places at once, because he/she may be doubling as cinematrographer/wardrobe/craft services/what have you, and everything needs to be done at once. In such situations, it's good to have more than one person who can offer decisive creative authority. On TA91, where microbudget perils endangered our shotlist on a pretty much daily basis, we were several times lucky enough to be able to split the crew into two units to shoot two scenes at once. Each director got to work on the stuff that they were good at, and eager to shoot, with the trust that the other would get the right shot over in the next room.
...Also, it belies the whole anti-democratic director-centric BS that so distorts people's ideas of what filmmaking is about. There's a popular illusion that the director's all-powerful vision spontaneously creates every single element of the movie. While it's much better to have a vision than to not have one, a vision is something you can have in the desert after eating hashish. It doesn't actually generate a movie. The movie happens because the director is good at his/her job of working with people who do certain things much better than he/she does, like act, rack focus, sew, etc. and sort of funnels all those skills towards a single end which is something like the director's vision. It's not an easy job, which is why you need directors, or else actors would just sort of act aimlessly about paperclips for 24-hour marathon handheld shots. But it is just one of the jobs on a set. Every job has a vision which in some way makes it into the movie. You know that feeling of seeing a movie which clearly had a strong vision, but that vision pretty much sucked? With a co-director you have some checks and balances.
...Lastly, and this is the thing I just figured out recently, a partner whose interest in the movie is the WHOLE movie, not just the individual success of a shot or a performance or a makeup job, is good to have when you're losing your mind. As painful as the working relationship can be, as difficult as it is to come to creative compromise (remembering that Hitler also had what could be called an "uncompromising vision," I think of compromise in the positive sense), it can be fruitful to have two people pressuring themselves to do nothing but make the movie a little better. The producer wants the movie to be finished and financially viable, and most actors want the movie to favorably showcase their talents. The director who is also the movie's producer, like me, often wants to give in to producerial concerns. And when I'm really wanting to do that, my co-director offers and idea for how to make the movie better. It's not always an idea we agree on, for sure, but it keeps that channel open. And it's an agonizing process, a lot of the time, but I don't think it's wasted time. In the end, it's much more true to the spirit of collaboration and co-existence that artistic ventures should be about, rather than this cover story of individual brilliant geniuses.