Re: 390 & 750 double pump ?
I doubt it, they are made to run with both lines. There are metering blocks on the front and rear of the carb and the front and rear work independant of each other. If there is a fuel balance tube from the front to the rear it might be done, but I doubt it.
If you are going to use that carb just for cruising, all you can do is keep a feathers touch on the pedal!!