Any 2 or more authorities may join or cooperate with one another in the exercise if any or all of the powers conferred for the purpose of planning, undertaking or financing a redevelopment project or projects located within the area or areas of operation of any 1 or more of such 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 chapter 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.