Your thinking is right in line with your base tune up. You doubled the available throttle bore area so the signal to each throttle bore is actually less. It does make a difference how you set the carbs up, progressive or 1:1 with how you’ll tune it. If you close up the secondary carb and idle on the primary obviously the signal will be the same as one carb. And the opposite is true if you’re idling on both carbs.