Charlie Jeffries made superb anglo concertinas. But other manufacturers were offering Duet concertinas and doubtless Jeffries received inquiries as to what had to offer in this system. Instead of making instruments in the established Crane (Triumph) or McCann systems, he created one of his own that was derived from the anglo. The resulting system splits an anglo row into two parallel rows, one of which plays the 'press' notes and the other the 'draw'. The resulting instrument is fast and compliant in keys that are close to 'home' but which has widely varying fingering nd button layouts as the keys venture further away.

As with many duets, Jeffries Duets were often large. They were made in limited quantities as most of the Jeffries output was still Anglos. Charlie Jeffries began to make them in the early 1920s and his sons continued to make them until the company ceased operations. The instruments were made to the highest standard of the time and the tone color is typically Jeffries with great carrying qualities.

Players of the Jeffries Duet are rare because few instruments were made and because there was no tutorial material or system to help players master the instrument. Only one recording of Jeffries Duet playing has been made, The Ramping Cat, by Michael Hebbert, which was released on the Free Reed label in the 1970s.