Using a stand mixer fitted with the whisk attachment, beat the butter, shortening, and salt together until very light and fluffy. Scrape the sides and bottom of the bowl with a spatula. Add the cream, vanilla, and almond extract and mix until combined. Slowly add the powdered sugar until all of it is mixed in. Scrape the sides and bottom of the bowl again. Add the lemon juice and whisk the frosting on high speed for 5 minutes. After five minutes, switch the whisk attachment out for the paddle attachment and beat on high for 2 more minutes. If you want to color your frosting, you can divide it into small bowls, and mix different food colorings into each bowl or make the whole batch of frosting one color.