Ok, I'm suggesting you look at the problem differently.
I think your problem is a flow problem.
Remember the engine is perfectly happy idling with one carb, it will also run all the way up to full rated HP with one.
So, at idle each carb is passing only 1/2 the air through the venturi. That means the vacuum pulling the gas through the low speed jet is much lower. A very minor difference in the construction of one carb or the other can be the difference.
And yes, your Uni-sync will not work right in this case.
There are 2 things you can try:
Option 1: With the linkage between the carbs disconnected, on one, back back the throttle plate fully out so the butterfly is fully closed. Open the screw on the other to set the idle to the proper speed. Then adjust the idle mixture screw on that carb.
Now back off that throttle plate screw to fully closed, open the one on the other carb to the correct idle speed and adjust that idle mixture.
Now forget about the mixtures and open the throttle plate on the both and adjust the balance at the desired idle speed. A couple hundred rpm's above the stock idle might be a good place to go for. Use the sound of the air flow to set the balance (length of fuel hose held to the ear and over each carb. back and forth till the sound is the same).
Option 2, If you have set the air flow so the carbs are balanced and the linkage is locked together, set the idle speed to twice the factory setting. Now adjust the idle air screws. Each carb should have the air flow of an original idle and the adjustment will work as designed. NOTE: the adjustment of one one carb will only have 1/2 the effect of a one carb system so you have to listen or watch the vacuum gauge very carefully.
Good luck.
