Historically the merger of one congregation with another has occurred out of weakness. Its purpose has been to create a merged congregation to live longer than the two might have. Many congregations continued their decline until within five years they were the size of the larger of the two at the time of merger.
In recent years new more proactive models of merger have arisen that have been more successful. Among the factors of success is the creation a new culture united around an empowering vision. This is more likely when the merger is of three congregations rather than two.