Devils and Angels
This is a bass trombone duet Michael Lake originally wrote for the Gerry Pagano and Doug Yeo duo album called Fratras. But it is more than that.
As you listen to the album track, mid-way through, another trombone (Michael Lake on alto in the recording) improvises over the two bass trombones. At the end of the improvisation, that third trombone assumes the lead part in what soon becomes a four part harmony, and then builds into a five part harmony. All this last only 8 bars before turning back to a bass trombone duet. In the last 6 bars, there is a short improvised section which, as a point of interest, Doug Yeo played a serpent!
Do you need all these extra horns? No. This entire piece can be played by only two bass trombones. The recorded track takes care of the harmony in those places where additional trombones play and the improvising occurs. Just decide which two parts you wish to play. Consider the bass bone playing chord roots to be optional.
Regarding the entrance of the two bones at the beginning, listen for the audio cue. Listening carefully to the track is the secret to playing this piece of music.