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That depends, and no one can answer that with certainty. The room where you have your incubator, may be more humid than where I have mine. Vica versa, your room may be warmer than mine, or colder. Whether you use one plug or two is going to depend on the individual environmental circumstances of your incubator and room.
ETA: Run your incubator for a week or two before you set eggs in it. Check it as often as possible, and write down your findings each time. This will help you to establish a pattern, and will identify when your room is the warmest, when its the dryest, when you need one plug, or when you need two.