You can block off one carb if you don't mind one end of the engine running a little leaner that the other. That problem can be minimized, but not totally eliminated with some jet size tinkering Use a piece of soft aluminum sheet metal instead of a base gasket and dimple the center of it down so the throttle plate on the dummy carburetor doesn't hang up on it. Also, block off the fuel line to the fake carb. The mouth-breathers at a "show and shine" will still marvel at how cool your ride is, and you'll have a lot better overall performance.
Overall, it's not a good idea to run a carb that far off center, particularly if you want the heat riser to work as designed. Tinkering with the way an engine was designed is very seldom successful, unless you're willing to do your homework first. It always involves a lot more work than just bolting a bunch of mismatched parts together.
Jerry
