A double concerto is a concerto featuring two performers, as opposed to the usual single performer, in the solo

Solo (music)

In music, a solo is a piece or a section of a piece played or sung by a single performer...

 role, whether these two performers' instruments are of the same type or different.

The best-known double concerto is that written by J.S. Bach

Johann Sebastian Bach

Johann Sebastian Bach was a German composer, organist, harpsichordist, violist, and violinist whose sacred and secular works for choir, orchestra, and solo instruments drew together the strands of the Baroque period and brought it to its ultimate maturity...

 for two violins in D minor

D minor

D minor is a minor scale based on D, consisting of the pitches D, E, F, G, A, B, and C. In the harmonic minor, the C is raised to C. Its key signature has one flat ....

.

Although there exist concerti with more than two solo parts for the same instrument (e.g., Antonio Vivaldi

Antonio Vivaldi

Antonio Lucio Vivaldi , nicknamed because of his red hair, was an Italian Baroque composer, priest, and virtuoso violinist, born in Venice. Vivaldi is recognized as one of the greatest Baroque composers, and his influence during his lifetime was widespread over Europe...

's Concerto for Four Violins in B minor, catalogued as RV 580 and transcribed for four harpsichords by Bach as BWV 1065), the terms "triple concerto", "quadruple concerto", etc., are seldom used.