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1 on top and 1 on the bottom? I would think it would give you more control and more air flow. The Weber kettle grills are set up that way and work great.
I would thinks as long as the bottom vent is below the charcoal pan it would supply enough air. (off to the side so you can reach it) And one on top. Give it a try and see if your going in the right direction. If it works your done. Or decide to add another vent or 2. Even if the extra vents are not what is needed you can keep them closed.