53 Pa.C.S. § 2348

Whenever a regional planning commission has been or is being established to serve the Pennsylvania portion of an area which, for planning purposes, constitutes a logical region as approved by the State Planning Board and which extends beyond the boundaries of this Commonwealth, the commission may admit to membership municipalities that are part of the same region but located in other states. Municipalities may participate, through membership and financial support, in commissions that have been or are being established in other states when the municipalities are part of the same region served by the out-of-State commission.