Progressive-linkage 2-barrel carbs open one barrel ahead ot the other, like the primary and secondary venturis on a 4-barrel. One barrel will have the idle system incorporated into it, with the fuel mixture screw and the idle speeed adjustment while the other one is for high speed only. It usually opens very quickly as the linkage approaches full throttle. A lot of people who can't or won't make the original carb work on a stovebolt engine like to swap over to the Weber-type progressive 2-barrel carb.
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!