Here is a description of Warming Up Together from the composer, Andrew Friedrichs:

Warming Up Together is a book of duets that are designed to provide a musical context as well as a focus on specific areas of technique. There are eight sections in this book that correspond to what I feel are the most fundamental components of trombone technique: tone (long tones), flexibility (slow slurs, wide slurs, and fast slurs), range (high range and low range), and articulation (slow articulation and fast articulation).

The idea for this book came out of my own warm up. I was trying to develop a warm up routine that was a series of technical categories rather than a series of exercises, but I also wanted to have the opportunity to be able to play it with someone else. As a trombone player and teacher, I have made specific choices within the duets based on technical challenges that Ive observed in my own playing and in the playing of students and colleagues. As a composer, Ive focused on addressing various styles in order to keep the duets fun and fresh while allowing the musical contexts to be as far reaching as possible in their application. As a not-to-be-taken-seriously backrow member of the orchestra, Ive thrown in the critical element of terrible humor by way of pathetic jokes and ignominious puns.