Mozart's Sinfonia Concertante in E-flat, K. 364, which was given one of its relatively rare performances Saturday night at the National Bureau of Standards, is twice as appealing as any of his violin concertos. He composed it at a more mature stage in his short career -- four years after the violin concertos -- and wrote into it two soloists for the price of one.

To the solo violin in this work, Mozart added a solo viola -- an instrument he loved to play because, as he explained, he liked to be "in the middle of things." Not many other composers have followed his lead in giving the spotlight to the viola, although Brahms took up the challenge of a concerto for two soloists successfully in his Double Concerto for violin and cello. Anyone who appreciates the dark, rich flavor of the viola must treasure K. 364, not only for its musical qualities but also because of its loving attention to that beautiful, neglected instrument.

Saturday night's performance, with violinist Alexis Galpe'rine and violist Miles Hoffman as soloists and Piotr Gajewski conducting the Montgomery Chamber Orchestra, did the music full justice. The dialogue between the soloists and between soloists and orchestra was beautifully paced, balanced and phrased; the music breathed naturally and spoke eloquently.

In the orchestra's second season, Gajewski has doubled its schedule from four concerts to eight -- a policy that apparently has found favor with audiences, if one can judge by the substantial attendance Saturday night and the enthusiastic applause. His programming is as fresh and well balanced as his conducting, with a strong emphasis on music of recognized quality that is seldom heard.

Besides the Mozart (which may be somewhat neglected because it requires two soloists), Saturday night's program included Faure''s gentle Pavanne, Op. 50, and Bizet's youthful Symphony No. 1 in C, given an interpretation that searched below the music's brilliant surface for its deeper qualities of emotional communication.