"Duo piano teams are a 20th-century phenomenon," says Steven Gordon, half of the husband-and-wife team of Steven and Nadya Gordon.

Piano duets (two pianists on one piano) and duo pianos (two pianists playing two pianos) have been around since the invention of the instrument, but, according to Gordon, mostly for the entertainment and amusement of the participants -- two siblings (or better, a shy, young romantic pair) staidly playing at the ornate mahogany upright in great-grandmother's Victorian parlor, or the master Liszt sharing a soire'e with a pupil. Great teams such as Vronsky and Babin or Gold and Fizdale were well known within sophisticated circles in the 1940s and '50s, but never approached the popularity of most solo artists.

*This may be changing, largely due to the influence of two young, charismatic sisters from France, Katia and Marielle Labe que, who will be performing with the National Symphony Orchestra Friday at Wolf Trap. It is the third stop in an American tour that began with a "Tonight Show" appearance and a recital at the Hollywood Bowl.

In performance the two are stunning. In addition to formidable pianistic skills, they have distinct personalities that are captivating: 34-year-old Katia is outgoing, demonstrative and athletic at the keyboard, while her younger sister, 32-year-old Marielle, is quiet and composed.

While their unprecedented popularity is calling attention to this often neglected ensemble, it may be that their most lasting influence will be in repertoire. Labe que concerts range from Bach to Berio to Gershwin to Joplin. And that is no accident. They recognized early in their careers that the repertoire for duo piano was tremendously limited -- a sample or two from a few composers and many transcriptions of orchestral music.

Katia Labe que says people "always hear Rachmaninoff suites," and that with audiences today they can be "more adventurous in repertoire." Sometimes accused of being headstrong, they Labe ques insist on playing contemporary "serious" music (Berio and Messiaen, for instance) or music that skirts the classical tradition, such as ragtime, at every recital.

The works presented "always have connection," insists Katia. For instance, they will pair Bach with Stravinsky, Spanish duos by Albe'niz with rags, or Ravel with Gershwin. The repertoire for orchestral dates is more limited, but the enjoyment of concerto playing -- Mozart's duo piano concerto will be featured with the NSO -- makes up for it.

"I love the orchestra," says Katia. "It is a special pleasure for us."

Being sisters fits right in with the duo piano tradition -- most great teams of the past have been closely related, by marriage or otherwise. And not without reason: Duo piano is arguably the most difficult instrumental ensemble to perfect. Since the piano is a percussive instrument with a strong attack, even the tiniest differences in timing will be painfully obvious to the listener. Perfection takes time; even at their ages, the Labe ques have been playing together for 25 years.

Steven and Nadya Gordon also have been playing together for 25 years. Performing about 60 concerts a year (they were at the Library of Congress this spring), they travel around the country with their two Bo sendorfer Imperial Grands. That's right, with them -- in a special temperature- and humidity-controlled trailer. Gordon says you just "cannot find perfectly matched pianos" even in most major halls, much less in the heartland. And rather than risk shipping problems, the Gordons keep their pianos where they can find them -- a trailer hitch away.

Rather than moving into the popular realm to increase their repertoire, they add new music by contemporary American composers such as John Heggie and Bruce Stark. Gordon believes the growing interest in duo pianism is finally making the medium attractive to composers.

The Gordons met at a competition. But Steven, immediately attracted to Nadya, had a problem -- he won, and she wouldn't answer his calls for a year. Fortunately, the next year Nadya won the same competition (this time the prize was bigger, which helped) and over the next 10 years they dated frequently while maintaining separate concert careers -- "we were going to be solo stars." And all the while they played piano together for their own pleasure.

Then came marriage. "For the first six months every time I was in Oregon, Nadya was in Florida." The solution was obvious and fortuitous, and since that time (15 years ago) they have concentrated exclusively on the two-piano repertoire, although Steven does occasional solo recordings.

After 25 years the Gordons have an almost mystical communication. Starting a piece, the most difficult part for two performers almost 18 feet apart, requires neither nod, downbeat nor one-two-three-GO.

"Her eyes just look in a certain way," says Steven, and they begin.