She bopped and she spun, she craned left and right, but the young girl in Row T didn’t know who to listen to. On the left of the stage a lithe, bouncy fiddler sang with a limber, white-gold tone. Next to him was

an assertive violist, the work’s harmonic puppet-master. Unfolding was Mozart’s Sinfonia Concertante for violin and viola, the repertoire’s most beloved double concerto of its kind, and the best explanation for the proliferation of stereo sound. The performers: violinist Christian Tetzlaff, violist Tabea Zimmerman, and conductor Bernard Labadie.

It was Tuesday evening, at one of those rare all-Mozart concerts at Mostly Mozart, and we had already heard a swift if untidy performance of the “Don Giovanni” overture, conducted by Mr. Labadie, the director of the early-music band Les Violins du Roy. Later we’d enjoy a luminous, more balanced reading of Mozart’s Symphony No. 39. But it was clear to me from the excitement of the girl and from the looks of hypnotized happiness that peppered the audience that the main event of the night was this dialogue between two of Mozart’s most favorite instruments.

The second movement of the fast-slow-fast concerto, which premiered in 1779, is thought to evoke Mozart’s loss at the death of his mother. But the outer movements are radiant flights of bright, E-flat major joy, and to render the viola’s muted tenor more sunny, Mozart wrote the viola part scordatura – a step up the scale. On Tuesday the ensemble made this difficult work sound a simple pleasure.

The Sinfonia Concertante tends to be performed in stately, virtuoso manners. String players often attack the first movement like epic Beethoven, to make the middle movement melodramatic, and to let the springs of the last movement loose like a Paganini Caprice. Tuesday’s duo took a more original approach. They played the runs of the first movement with improvisational spirit.

Every 16th note, for instance, didn’t enjoy the same rhythmic value, but whatever was taken away from one was added to another, so the end total remained correct and the form of the piece intact. The performers accomplished this feat in sync, allowing the brisk tempo to fluctuate while remaining together. They would also articulate a clipped passage more legato than usual or hesitate on a middle-run note that others would barrel through.

In this duo’s hands the second, grieving movement wasn’t overly sad or dark. It was solemn but life-infused, moving more quickly than usual. The cadenza, which allowed the players to elaborate on the work’s themes without orchestral accompaniment, was particularly poignant: an operatic aria made of fast and slow interjections, topped with a perfectly timed yet off-the-cuff-sounding trill in which both musicians kept the turns wonderfully tight. This was planned, to be sure, but it never sounded like it.

The hopping rendition of the last, quick movement brought it to an appropriate close. Both players produced raw, gutty string chords before unleashing a barrage of descending triplets. And amid the chromatic deceptions and contrapuntal retorts, I began to understand why some people say listening to Mozart makes children smarter. In any case, it made the little girl in Row T a whole lot happier.