Reading your latest post would indicate that the front carb was most likely contributing excess fuel to the front cylinders thereby en-richening their mixture. If that in fact is true I would also expect to see have seen some difficultly in setting the mixture screws.

I have been running the mechanical six packs on various motors for about 40 years and will offer the following:

The secondary carbs are just that, secondary.

Think of a 4 barrel with an extra set of secondaries.

The main idle and low speed operation is mainly controlled by the primary carburetor. A very small amount of fuel is contributed by the secondaries during low speed / idle mainly to keep the fuel from going stale in the bowls.

I have always set the the secondaries idle screw about 1/8 to 1/4 turn open on both carbs trying to egt them as equally set as possible. I have then used the center carb for the rest of the low speed /idle operation.

With a bumpier cam and an automatic they can be opened a bit more if needed to get the primary blades in the correct relationship to the idle, off idle transition. Once the secondary adjustment if equalized the linkage should "SLIP" into both carbs with no pressure.
keep us posted