Sounds like a tough one. For the slides, it not really that they open at the same time, but where they are in relation to each other when the slide cutaway reaches the top of the throat. Usually just advance the throttle until the first one reached the top and make sure the other one gets there at the same time.

Other things to look at if you hadnt already,

Pulse line on your fuel pump. Make sure that it is the correct material (this is different than standard fuel line). Sometimes it can get soft and flex the tube vice the diaphragm in the pump and make the pump behave badly at times or lower its capacity.

Pulse line length, should be as short as possible

Pump diaphragm, these can sometimes leak enrichening the pulse side of the engine or if it pumps air into the fuel pump cause other issues.

If you have clear carb fuel lines installed do a ground run and make sure you dont see any bubbles coming through downstream of the pump.

You mat have already done this, but always worth swapping the carb positions to see of the problem moves with the carbs.

Has the engine ever been through a rebuild? If so, might be work finding out more of those details. Not sure if an improperly indexed rotary valve would behave in this manner. Just odd that one EGT is higher than the other by 100F. How does it look at other throttle positions? Say from 4500 through 6500.

Yep, float height could be a player as well, as well as needle and seat.

You might have noticed that there was a float recall recently for those carbs. Make sure your floats are still floating fully and that you have the newest floats. I had an issue with that problem. New floats helped a great deal. Also installed new needles and seats.