OK- - - -I see part of the problem- - - - -the tubes between the manifolds and the heater plate are far too small, unable to flow enough volume of exhaust gas to keep the intake plenum hot. Try to increase the tubing size to 1/2" (12 MM maybe?) so enough gas can flow under the carb to keep the mixture atomized. If the Zenith carb was performing OK with the stock exhaust there's no reason to believe it can't function as well with duals. Unless you can find some VERY small carbs for a dual setup, the bog will get much worse. Most people seriously over-carburete a twin carb engine.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!