What does "double in brass" mean?

Serve in two capacities, as in In this company everyone is asked to double in brass occasionally. This expression was originally used in the traveling circus, where, for example, a clown was also expected to play an instrument in the street parade. The original allusion, however, is to an instrumentalist who plays more than one instrument in an ensemble, a practice particularly common among players of brass instruments. [Late 1800s]