Presidential Airways announced yesterday that it will join forces with American Airlines to more than double the capacity of its new mid-field terminal at Dulles International Airport.

The decision to share facilities -- and possibly to coordinate market strategies -- with American comes only two months before United Airlines, the nation's largest air carrier, is to begin service at Dulles, and at a time when competition at the nation's fastest-growing airport has suddenly become intense.

Presidential also announced yesterday that it will serve three additional markets, and that it has created the Washington Air Corp., a holding company that will operate a new carrier, "Presidential 1," as a feeder airline for Presidential from several markets in the mid-Atlantic region. The new carrier intends to begin operation of four turboprop aircraft in April.

"We now provide more service to more destinations than any other airline operating at Dulles," said Harold J. Pareti, Presidential's chief executive officer, who said that the airline will soon begin flights to Atlanta, Fort Lauderdale, Fla., and Allentown, Pa. "And we felt that it was best to move forward as a team with American."

Clearly, one of the reasons that the five-month old Presidential wanted to work with American Airlines is that United's impending arrival could instantly disrupt their new low-cost, no-frill service to many cities.

United announced in December that it would initiate a major Washington-based East Coast expansion, with more than 50 daily departures from Dulles to 24 destinations. Its enormous financial and scheduling strength will make it a tough competitor for newer airlines, such as Presidential and New York Air.

Pareti said yesterday that he approached American executives about two months ago to see if they were interested in sharing the cost of extending Presidential's new terminal and in coordinating route schedules.

The agreement, which will require the approval of the Federal Aviation Administration, calls for the construction of an $8.8 million, 117,000-square-foot addition to the terminal Presidential has just finished. Under the plan, the airlines will allow each other access to their gates. They will also share baggage and fuel facilities.

The terminal expansion will enable customers of both American and Presidential to use each other's gates without having to ride the mobile lounge that transports passengers from the main Dulles terminal to the airline facilities on the airfield.

As a result, travelers who arrive at an American gate and depart from Presidential will save the more than 20 minutes necessary to ride to the main terminal in order to switch planes.

The move also comes at time when airlines throughout the country are sharing facilities and marketing strategies with more frequency. "This is a trend that you see more clearly every day," said George James, president of Airline Economics, a firm that follows the airline industry. "There are a lot of advantages of hooking up like that. Obviously, the competition that United will create is one of them."

Although the use of Dulles is growing at a rapid rate -- 5 million customers passed through the terminal in 1985, twice the number of the previous year -- Presidential reported yesterday that its passenger total fell by 7 percent in January from the previous month.