Preheat oven to 325 degree. Grease and flour a 10 inch tube pan. Mix together the flour and baking powder. In a large bowl, cream together 1 cup butter, 1/2 cup shortening (I use Crisco), and 2 cups sugar until light and fluffy. Beat in the eggs one at a time. Beat in the flour mixture alternately with the milk, mixing just until incorporated. Stir in 2 tablespoons rum, key lime zest, 2 teaspoons key lime juice, vanilla extract and lemon juice. Pour batter into prepared pan. Bake in the preheated oven for 90 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Make the glaze by combining 1/4 cup white powdered sugar, 1/4 cup butter and 2 tablespoons key lime juice in a small saucepan. Let it boil for a minute or two, stirring constantly. Remove from heat and stir in 3 tablespoons rum. Allow the cake to cool in pan 10 minutes. Turn out on wire rack. While warm, prick the top of cake in many places with a toothpick. Pour the glaze over warm cake. Cool completely before serving.