I think Jim is right on this one, the air is moving so slow when both throttles open, it can't pull any fuel in with it. Try opening the idle mixture screw some more. If you can get it to idle really rich, you might over come the off idle lean. If it does work, then you are going to have to figure a way around the rich idle. My dual mono-jet carbs were way to big for the 250, one was plenty, it ran rich all the time. Full throttle felt really good, as did normal driving, problem was you could almost see the fuel gauge moving.
I now run dual W-1 Carters, a much smaller carburetor. Fuel mileage went way up along with all the problems of the Mono-jets.
What type of linkage are you running? Maybe you could go with a progressive type with the back carb slightly behind the front with a slightly quicker opening ratio. At highway speed both would be open the same or close to it, but off idle one would lag behind to off set the airflow of both opening at once.
