Ditto- - - -with nothing to push against, whichever piston meets the least resistance will move while the other one stays stationary. With the drums installed and the adjustment done, neither shoe will be able to travel more than a few thousandths of an inch without one of the linings contacting the drum. Then the other one will move into contact and the stopping will begin. There won't be more than a fraction of a second's difference in the application of both shoes- - - -not enough to measure with any sort of accuracy.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!