As long as the M/C piston bore diameter and stroke is a close match to the original cylinder, the dual cylinder should perform the same. Hydraulic brakes are a simple application of physics- - - -a certain bore piston displaces a specific volume of fluid, and the leverage of the pedal provides the correct piston stroke. Dual outlets and stacked pistons inside the master cylinder just provide a safety factor in case of a leak instead of losing all the braking effect.
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!