Spray 2 loaf pans with nonstick cooking spray or line with parchment paper.
Cream the sugar, shortening, butter and lime zest together in a large bowl or in the bowl of a stand mixer for two minutes.
Slowly add in the eggs and continue mixing for 5 to 6 minutes.
Periodically scraping down the sides of the bowl.
In a medium-size bowl mix together the flour, baking powder, and salt.
Pour the milk and key lime juice in separate containers.
Alternate adding in a cup at a time of flour, and then 1/3 cup of milk, and a little bit of lime juice continue mixing together and repeat until all three ingredients are in the bowl and fully mixed.
Pour the batter equally into the two baking pans and bake in the center of the oven for an hour in 10 minutes or until a toothpick comes out of the center clean.
Allow the cakes to cool for 10 to 15 minutes on the counter.
And then invert onto a cooling rack for at least an hour before cutting.