Both cities maintain a fleet of vehicles and backup vehicles. Both cities employ drivers, dispatchers and mechanics. A manager has to allocate a portion of his time to oversee them. If the cities merged, the need for backup vehicles would be reduced and staff reductions would be possible. To taxpayers this is a better use of public resources than separately funding each system.
Public transit is only one of the many services provided by government.
And, public transit is not a huge expenditure, either. Imagine, then, the savings that can be realized in other aspects of local government, such as road building and maintenance. A merger of cities can also play a big role in economic development. In 1963, Nashville merged with its surrounding county areas. The population of the city of Nashville immediately increased to a size large enough to entice businesses looking for a new place to locate.