Preheat oven to 325°F.
Use butter to grease the loaf pans, then dust with flour.
Sift flour, baking powder, and salt into a mixing bowl and set aside.
In the stand mixer, cream together the butter, sugar, and shortening.
Add the eggs to the butter mixture one at a time, beating well after each egg.
Add about ⅓ of the flour mixture to the butter and eggs and mix until just blended.
Add half the milk and blend.
Add half the remaining flour mixture, and blend again.
Add the remaining milk, blend. Add the remaining flour mixture, blend.
Add the lemon juice and zest, blending one final time.
Pour the batter into the loaf pans evenly, giving them about 1-2 inches of space to rise.
Bake for 50-60 minutes, or until a probe thermometer indicates the internal temperature has reached 212°F.
Remove from the oven and let cool for at least 15 minutes. Turn the loaves out onto a cooling rack to cool until icing.
Make the icing by combining powdered sugar and lemon juice until the desired consistency has been reached. If it's too thick, add more lemon; too thin, add more sugar.
Ice the cakes using your preferred method, then slice and enjoy!