Although I have never worked on a project where there are multiple project managers, I think having more than one project managers could be useful in certain cases such as when development projects run astray/run into issue and help is needed or in non-IT companies, where there could be two project managers, one is from the business side and the other is the technical project manager. Although having more than one project manager can be useful at times, I think it should be used as a last resort when having one project manager proves to be insufficient and dysfunctional and this decision should be handled with caution.