With the 8 carb X3, it is a cross ram with a separate left and right plenum. Each 2 bbl carb sits right over the top of a port. I'm not sure, but it would seem that the a/f ratio of each cylinder would be most affected by the carb that is above that cylinders port. The carbs have aftermarket adjustable main jets, with a needle valve extending partly into each jet. So the point of the 8 O2 sensor test would be to equalize the a/f ratio of each cylinder, then dial in the most powerful overall ratio. THen when tuning for weather changes, turn each needle exactly the same. In this pic you can see the needle valves.