Any two or more authorities may join or cooperate with one another in the exercise of any or all of the powers conferred hereby for the purpose of planning, undertaking or financing a redevelopment project or projects located within the area or areas of operation of any one or more of said authorities. When a redevelopment project or projects are planned, undertaken or financed on a regional or unified metropolitan basis, the terms "governing body" and "community" as used in this article shall mean the governing bodies of the appropriate communities and the appropriate communities cooperating in the planning, undertaking or financing of such project or projects.