When you double or triple (or more) a recipe for cakes, cookies, muffins, quickbreads, and other baked goods, you should begin by following the multiplied recipe conversion as provided by HalfRecipe exactly. However, some recipes tune the amount of leavener to the size of the finished end product. For example, a recipe may have enough leavener to be used in a 9 inch round pan. If you're using a larger pan with a larger amount of, for example, cake batter, then you might need to adjust the amount of leavener down.
So, if you're doubling a recipe for a certain sized pan to make two cakes, double the leavener. If you're multiplying a recipe to bake a single cake in a larger pan, then you'll need to adjust the amount of leavener.