Cream together butter and shortening on medium high speed until combined and fluffy, about 3 minutes.
Add lemon juice and salt on low speed.
Then start adding sugar, one cup at a time. Then add meringue powder.
Add milk and beat on slow until combined and then on medium high until light and fluffy, about 5 minutes. If your frosting is still dry, add 1 Tbsp. milk at a time until you reach your desired consistency.