The expression “to double in brass” means: To work in two jobs in order to increase one’s income; to earn money from two sources; originally, and still, to play in the band as well as to perform.
The expression is used primarily by theatrical people, especially by old-timers, when referring to an actor who, for economic reasons, appears as two different characters in a play.
Undoubtedly, however, it originated in the circus, the American circus as developed by Phineas Taylor Barnum. And probably it was altogether literal in the early days of “The Greatest Show on Earth”, back, say, in the 1880’s.
Literal also in the one-ring circuses which hit the small towns today.
The equestrian, the aerial trapeze artist, the clown who can also play a cornet, trombone, or clarinet in the street parade and when not engaged in his act has a better chance of employment than the straight musician or the straight performer.
The expression, literal or figurative, seems to be unknown in England; at least it does not appear in the latest collections of British “unconventional English.”