New carbs??..

The advantage for having them hooked up 1 to 1 where both of them open up at the same time is much better throttle response. The disadvantage is it a little harder to get control of as the spring pressure is doubled on your gas pedal and makes it tough to feather easily on the street..
Hooked up Progressive you generally have about a third of the throttle on your first carburetor and then the second one kicks in and picks up pace in order to reach full throttle at the same time with the initial carburetor. Of course when you're working on that first third of a throttle you're only pulling on one carburetor so your pedals softer and easier to manage and it's less abrupt..
you can feel when you've made it to the second carburetor because now you're pulling the tension of 2 carburetor Springs with your gas pedal. It feels a little bit like the turbos kicking in right when you feel that second spring start pulling sort of the back tire lol....
Once I got used to the heavy pull of two carburetor Springs at once I enjoy the more crisp response of the gas pedal..