I agree, I think one of your throttle cables is in a different spot. One must be in a different slot changing the ratio of that throttle.

I have a vacuum gauge that I use for dialing throttle on carbs, and it is a flawless metering tool....
Yours to borrow if needed

I am no expert and never had to tackle this sort of issue but if I your problem in front of me I would use a ruler and start with one lever and measure the throw in maybe 1" increments. Set the throttle at 1" then measure what you have at the carb. Then 2" and go down the line and record what you got. Move on to the next throttle lever and compare the measurements that would tell me if the lever/ cable geometry is in the same and go from there. I would think it would be pretty easy using this method to sync the motors especially if you had to people and the person on the carb end payed attention to the carbs closely... When secondaries start to open ect. Ect. Maybe I'm way off base but that's what I would do.

If you wanted to get really technical about it you could use dial indicators...

I hear that the RCH (Red C-Hair) offers the finest measurement.

Can't offer anything more than has already been mentioned. Having my own issues with throttle travel and getting it back down to an idle below 1K RPM's with mine. Couldn't imagine the double-migraine of dealing with twins on blower motors. Good luck!

I had to start all over....so many points of adjustment on the set up I have... because of the spring oilers I had to shim everything up and and that screwed me all up. Both motors idle fine now and both carbs seems to opening at the same rate and equally. We shall see what happens on the water.
Thanx for all the ideas