A Project Manager can enable other Project Managers to access his project(s) as a Co-Manager. This is usually done so that the Co-manager(s) can cover during the Manager's absence, or for allocating part of the Manager’s workload.
A Co-Manager's My Projects level permission within the permission sets defines what level of access the Co-Manager has on the shared projects. The Co-Managers option applies to Project Administrator, Project Manager, Program Manager and Client Manager permission sets only.
Note: The Co-Manager will not be able to approve the timesheets of the users for that project, approvals will happen only through the Project Manager. Co-Managers have access to all the reports the project manager has.