[Mart]

For what it's worth, I tried progressive on one of those offset type manifolds and found it more easy to make it work well when I switched to running both carbs at the same time.
same jets in both carbs, functioning but lower number power valves.

Re the suggestion to block off the rear carb. Actually I think this would be a good idea in this case, purely for testing purposes.

I'd remove the rear carb and just run one in the central location. Get it dialled in and running perfectly. Stock jets, 51s and a stock power valve.

I'd then swap in the other carb with exactly the same spec. get it dialled in and running perfectly.

Only by eliminating the twin progressive factor will you prove that the engine is running well. Condensor? Sticking valves? plug wires? get it running like gangbusters on one carb first. It will probably go quite well.

Then, try fitting both carbs. Leave the settings as stock (to start with). Set them up running in parallel. (Not progressive).

If there is an issue, address it. But make sure both carbs are exactly the same. From my experience, they might want slightly lower number power valves, but definitely not NO power valves.

If it runs better with them both on then great. If not, well, it's up to you.

But definitely get it running strong with one carb first, to eliminate all the "noise" that may be effecting your results.

Mart.