Add the softened butter, shortening, vanilla extract, and salt to the bowl of a stand mixer fitted with the paddle attachment, and mix until smooth.
Mix in the powdered sugar a little at a time, very slowly, until it is all incorporated.
Add in the milk on low speed. Once the milk is incorporated, crank the speed up to medium high, and whip until the mixture becomes light and fluffy (about 5 minutes).
Once your frosting is looking light and fluffy, it's time to add the cream cheese. With the mixer still running on medium-high, add in your cream cheese a little bit at a time until fully incorporated. Whip for another minute or two.