In my experience, I have encountered two project managers in a project. The project managers responsibilities were split; one was responsible for overseeing development activities at suppliers and the other development activities at our company. This approach was successful and I believe the main reason is because their responsibilities were well defined. I agree with posts such as from ppp23 that having multiple project managers in one project can have drawbacks. Project managers have different personal characteristics, organizational and management skills which can be conflicting and incur delays in decisions. In addition, if their responsibilities are not well defined, none of them may take ownership of the project which can cause lack of support and compromise the project.