Six Duos for Trombones, Op. 50 by Friedrich Belcke composed in 1832 may be one of the first sets of duets specifically written for Trombones.
Belcke explains that the duos are all in the same key of B-flat and may be transposed into any key or clef, which adds a serious challenge for the performers beyond the initial intermediate level of the music. Check out the sample and test yourself....can you transpose up or down a half or full step at sight?
Niels-Ole Bo Johansen has edited these six duos so both parts can be shared with one score.
This music is available as an e-book on Amazon.
The soundtrack below is of duos 1 & 6.