Affleck confirmed in an interview with MTV that he and brother Casey are currently re-working the script by Dave Mandel, and that roles currently aren't solidified. He, Casey, or possibly even Damon could wind up as director.
Affleck: I don’t know how it’ll come together, acting and directing, but Casey and I have decided to write it together. Once the script’s kind of ready, then it’s like: Who’s doing what? What’s the time frame? Matt, me, Casey directing? Particularly for Matt, he’s had this extraordinary career where he’s worked with these great directors. He’s a great director who just hasn’t directed a movie.
That would be one hell of a directorial debut. Considering Damon and Affleck have been best friends for years, it would sorta ironic to see them playing two best friends who take their fondness for eachother to such an extreme degree. We haven't seen them really team up on a movie together since Dogma