In any case, probably the only reason we pay much attention to the genre today is because of a superb composition by Mozart, the Sinfonia concertante, K. 364, in E flat for violin, viola and orchestra.
The Wikipedia article on the sinfonia concertante form is not very good. It seems obvious to me that the genre is a holdover from the Baroque concerto grosso, though reinterpreted in the Classical style. In any case, the only important piece in this genre is the one by Mozart, but it's a good one.
The Sinfonia concertante in E flat is a unique piece with a unique sound. Mozart loved the viola and may have written the viola part in this concerto for himself to play. In any case, the sound of the whole orchestra is imbued with the timbre of the viola. Take the first chord for example:
The violas, divisi, are playing quite high double stops, in the same range as the violins, in their lowest register, with the oboes in their low register, the horns doubling the oboes and cellos. The effect is to the give the piece a characteristic sound, shaped by the viola. Rosen remarks that this is a "milestone in Mozart's career: for the first time he had created a sonority at once completely individual and logically related to the the nature of the work." [The Classical Style, p. 215]
Though the slow movement and rondo finale are very fine, the real weight of the piece is in the somber first movement. The themes are closely linked:
The solo parts are written with both brilliance and pathos. They often echo one another so closely that listening to a recording without the score you might at times be puzzled as to who is playing what. While the timbres are different, the instruments are handled very similarly. Here is the brief but effective cadenza at the end of the first movement:
This concerto represents a considerable increase in maturity in Mozart's writing and also is his final example of a concerto with more than one soloist. From now on he will compose only concertos for solo piano and orchestra, mostly for his own use. And that we will take up next week. In the meantime, here is a performance of the Sinfonia concertante in E flat with Gidon Kremer, Kim Kashkashian and the Wiener Philharmoniker conducted by a young Nikolaus Harnoncourt: