Bell Atlantic and Nynex Monday, as expected, said they plan to merge into one company with a combined market value estimated in excess of $50 billion. The merger will create the nation's second-largest telecommunications company, with 133,000 employees and revenues of $27.8 billion (1995). The new company, which will take the Bell Atlantic name, will be headquartered in New York and serve 26 million customers in the Northeastern and mid-Atlantic states. Raymond W. Smith, chairman-CEO of Bell Atlantic, will keep the same title at the merged company. Ivan G. Seidenberg, chairman-CEO of Nynex, will be vice chairman, president and chief operating officer.