In my opinion, multiple project managers for a single project is not an ideal solution. Multiple managers would involve multiple behaviors, acceptance on a single solution for a part of project is difficult, bringing all the managers to accept a unique point of view. In my past company, we had multiple managers to report on a project. They both were so different personality making the project team difficult to provide solution. One manager used to agree for the solution whereas the other disagreed. This caused project delays, confusion and no success. Eventually the project had to be called off and management later decided to assign a single manager for a single project.