Although I do not have industry experience, my (limited) experience in working with hierarchical groups leads me to believe that having multiple managers can sometimes be a recipe for disaster. Being a manager promotes a sense of responsibility. A project manager for the “X Development Project” knows that, at the end of the day, he is responsible for the deliverable “X,” and that he cannot blame his (or her!) subordinates for failure to do so. A singular manager also helps avoid conflict. When there exist multiple managers with equivalent authorities, fights between can be long and destructive, as there may be no direct authority figure able to intercede and force a truce.

The benefits of a system with multiple managers, in my opinion, are vague. Multiple managers may bring different strong points, yes, but the point of a good manager is to understand the project as a whole and assign subordinates to tasks based on their strengths. A good manager can manage a hierarchy of people with varying skillsets, assign those people based on where they are needed, and mediate any conflict that may develop between them. As Scott mentioned, under a top-level manager are sub-level managers. This hierarchy, when well-implemented, will provide the benefits of having multiple managers without the drawbacks.