Pianists Alan Mandell and Edward Mattos spent a lot of their time skating on thin ice last night at the Wolf Trap Barns. But they have been doing it since 1973, in more than 100 performances all over the world, and by now they know exactly how far they can go.

"A Party for George (And Ira, Too)," their Gershwin program, included a wealth of material that is the property of every cocktail-lounge pianist in the world: "Embraceable You," "Liza," "The Man I love," "I Got Plenty of Nothin' " and "I Got Rhythm," for example. And they played it in a style that any cocktail-lounge pianist would be happy to match, with glittering glissandi, lots of pepper in the harmonies, cute little tricks with rhythmic rushes and hesitations and a lot of expressive dynamic changes from loud, hair-raising growls to a muted whisper.

It tottered again and again on the brink of excess -- but it was perfect, exactly the way to play Gershwin's music, at least this kind of Gershwin, which raises the idioms of the American cocktail lounge to the level of high art. According to the available evidence, on records, film and piano rolls, the two pianos on the Wolf Trap stage last night were very close to what Gershwin would have done if he could have played two pianos simultaneously. He was a phenomenal pianist, but that was a bit outside his reach, although he certainly would have liked to if he could.

It is hard to imagine anything more American than this program, which included two-piano arrangements of melodies from "Porgy and Bess," a dazzling performance of the bluesy Second Prelude for Piano, played by Mandell, and a very virtuoso performance of the "Rhapsody in Blue" based on the composer's original sketch for two pianos. But the show actually originated, at the request of the Soviet Ministry of Culture, in Moscow in 1973, for the observance of the 75th anniversary of George Gershwin's birth. Gershwin's parents were both born in Russia, and he is loved and honored there as a native son. Other places where the program has been happily received include Rumania and Indonesia, where many love Gershwin not as a native son but as an artistic citizen of the world, a man fluent in a universal language.

Last night's program worked extremely well in the rustic American setting of the Barns, partly because Gershwin is primarily American, but mostly because the acoustics are superb and the magnificently skilled pianists have mastered the material in every detail.

Ira, who wrote the lyrics for many of George's best songs, was well-treated by vocalist Stephanie Nakasian, particularily in the clarity with which she pronounced every word, the precision of her rhythms and the expertise with which she phrased the music. Her tone could have been warmer and more varied, but it did improve significantly in the later songs. At the piano she had Hod O'Brien, whose solo moments were brilliant and whose accompaniment was highly skilled. The program will be repeated at 8 tonight and 2 tomorrow.