Frequent congestions that occurred at the border between PJM and MISO inevitably forced the two regional planners to find a remedy based on transmission solutions. This triggered the two RTOs to develop a joint methodology that would be acceptable to both entities. Such a methodology for promoting cross-border transmission at the seams of two adjacent electricity market systems is presented. This method is based on the economic criteria that are applicable to both entities individually and jointly. The method is also novel compared with the current methodologies for planning both market-based and reliability-based transmission projects within each market. This method is applicable to other electricity markets or transmission regimes where such cross-border transmission expansions are desperately needed.