The violin and cello don't often go out alone, and even more rarely do they go out together. On Monday evening, violinist Juliette Kang and cellist Thomas Kraines explored music for both instruments in solo and duo works. The concert, which was devoted mostly to music of our century, was presented by the National Chamber Orchestra at the F. Scott Fitzgerald Theatre in Rockville.

A rather enigmatically titled work by David Vayo--it was called "Piece for Violin" in the program and "Music for Violin" in the composer's program note--was a world premiere. Vayo wrote the work specifically for Kang, a talented young violinist who has two prestigious international awards (the Indianapolis and Yehudi Menuhin competitions) to her credit.

Vayo builds a large arch, beginning with the simplest and most austere elements, mournful lines slowly sliding around each other, breaking into fragments and coalescing again. He adds to this a haunting, wordless vocal line--sung simply by Kang--which plays with our perception of the violin. Is it a lyrical, soliloquizing instrument, a surrogate for the human voice? Or is it a more mechanical thing, an accompanying device for the real voice?

Vayo's music maintains a tension between these two perceptions, building to a climax that owes much to the purely instrumental virtuosity of solo violin works by J.S. Bach and the Belgian composer Eugene Ysaye. Yet despite the dazzling instrumental fireworks, the piece ends darkly, with the violin disappearing into silence with a particularly human vulnerability.

Vayo's control of his unorthodox elements--he also uses whistling, ferocious pizzicati and wraithlike harmonics--is impressive. He has worked out his ideas thoroughly in the compositional process; nontraditional elements are used purely expressively, not to alienate the listener.

More traditional were works for violin and cello by Zoltan Kodaly and Ernst Toch, and a snippet of Ravel played as an encore. Kodaly's Duo, Op. 7, is a monument of the small cello-violin repertoire, in many ways even more impressive and lovable than Ravel's duo written years later.

When Ravel broke down the boundaries between the two instruments, he fused them with an almost erotic intensity; Kodaly, on the other hand, maintains their individual voices, passing ideas effortlessly between cello and violin while never completely suppressing either player into a purely accompanying mode. In the second movement, they weep for each other, and in the third, they goad themselves into a kind of Gypsy fiddling contest.

Kang and Kraines play well together, though one wished at times for more freedom and humor. They avoided opportunities for levity, like the rather jaunty third movement of the Toch Divertimento, Op. 37, No. 1, and they might have found a bit more fluid and conversational back-and-forth in the Kodaly. But they have devoted themselves to scores that need more frequent hearing, and nobody could come away from Monday's performance doubting the worth of the music they performed.

CAPTION: Juliette Kang was soloist and muse for the premiere of David Vayo's "Music for Violin."