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I am smoking two racks of pork ribs tomorrow. One is 7 pounds and one is 6 pounds. Do I combine the weight and smoke at that time or do I smoke them at their individual weight?
Treat each as if it's on the pit by itself. Don't worry about time, cook to IT. You probably will experience a longer cook because of the added mass in the smoker.
The number of racks of ribs in your smoker will not affect cook time provided your pit is kept at a consistent temp. At most you might use a little additional fuel maintaining that temp. You will not even notice it as a factor.