I never worked on a project under multiple managers, but I would imagine that it would be harder than having one main manager. Although is all depends on the project and managing environment, things may get a little complicated, as multiple managers may have different commitment and responsibilities which would hinder the project flow and the timeline as well as leads to some communication concerns, multiple planners would also be an issue if each manager have a different point of view and different expertise which could cause some conflicts. Advantages would include having wider thoughts and different point of views and counter opinions which would influence the project positively in the long-term.