The new fall ballet season in Washington, which goes into full swing early next month, will be launched this year by not just one but two major visiting troupes from New York. The juxtaposition could prove to be a critical test of the Washington ballet audience.

The Dance Theater of Harlem has been booked into the downtown Warner Theatre for a three-week period starting Oct. 3 and ending Oct. 19, it was learned yesterday. The company will be running virtually neck-and-neck with the New York City Ballet, which plays the Kennedy Center Opera House Oct. 2 through 21 on a ballet subscription series which has been set for some time. The irony of the situation is that DTH founder-director Arthur Mitchell was a longtime principal of the NYC Ballet and a protege of the latter's director, George Balanchine, whose choreography is amply represented in the DTH repertoire.

The coincident bookings weren't arranged that way. Sam L'Hommedieu, who set up the DTH engagement for the Warner and who has previously been involved mostly with rock music groups, was unaware of the NYC Ballet performances until a few days ago. Arthur Mitchell, who was in London with his company completing a highly successful overseas tour until last Thursday, also didn't know about the potential Balanchine conflict.

When Mitchell learned of the situation, he discussed it with his company board and then called Balanchine, who unhesitatingly approved of the dual appearances.

"It will sort of be like a father-and-son act," Mitchell said in a telephone interview from New York. "The fact is, we're a very different kind of company from the City Ballet, and out audience tends to be different also. We know we have many friends in Washington from the company's previous visits, and I think it's going to work out fine."

The Dance Theater of Harlem, established in 1969 as a proving ground and showcase for black classical dancers, last performed in Washington in the summer of 1977 at Carter Barron Amphitheatre, where it drew substantial crowds. At the 2,000-seat Warner, the company will present four separate programs, and among the 15 ballets will be seven Washington premieres, including Mitchell's own "Manifestations" and "The Greatest," as well as Royston Maldoom's "Doina," Robert North's "Troy Game," Carlos Cravajal's "Shapes of Evening" and others.