Two co-founders often think they can share the executive responsibilities as co-CEOs. Oftentimes, two co-founders think it is a good idea to share CEO responsibilities as co-CEOs. Or, it requires one of the co-CEOs to back down and agree to the other CEO (usually with the louder voice and personality winning). At that point, you don't really have a co-CEO structure at all, with one person needing to control all decisions. There are examples where co-CEOs have worked together perfectly — think of the Google founders (Sergey Brin and Larry Page).