FWIW, with the standard cable setup I have found that the carb slides can be "in sync" at low/idle speed and out of sync at the top of the throttle bore. IOW, being in sync at one point doesn't mean they are at another point.
Also, as noted, the same setting on both carbs is not necessarily the correct setting for best running. The way I set up idle on my Commando is to ground one plug wire so it is running on one cylinder. I adjust the airscrew/throttle stop on the working cylinder to achieve the lowest idle speed that it will maintain. Then I do the other cylinder the same way, adjusting to the SAME RPM. When I reconnect both cylinders, the idle ends up pretty much where it should be. That being said, I have not observed any major difference in the airscrew settings - both are close to the spec - 1 1/2 turns out - with less than 1/8th of a turn difference between them.
Re carb jetting/settings - in my performance/competition engine-building days, we often found that jetting had to be different for multiple carbs sets. IOW, though I never worked on a Norton engine for that use, it would not have surprised me that maximum power/best running was achieved with different jets/settings on each carburetor.
What kind of air filter are you using? If separate for each carb, like a "sock"-type, it is possible that one is more restrictive than the other. I ran velocity stacks on my Commando for a while and discovered that when the jetting was correct - both plugs looking essentially perfect at a WOT plug chop, adding one of those screen "filters" (to keep the birds out)
changed the plug reading from "right" to sooty/too rich.
