The nation's two biggest flight attendant unions announced a merger, saying their combined membership would include almost half of the flight attendants in the United States. The planned merger would fold the smaller Assn. of Professional Flight Attendants--representing attendants at American Airlines--into the bigger Assn. of Flight Attendants, affiliated with the AFL-CIO. The new union, with headquarters in Washington, would have 35,000 members, officials said, and become the Assn. of Flight Attendants International.