Beat butter and shortening at medium speed with an electric mixer until creamy. Gradually beat in 1 package sugar and meringue powder; add liqueur, vanilla, and salt, beating until blended. Gradually add remaining powdered sugar alternately with 1/2 cup whipping cream, beginning and ending with sugar. Continue to beat at medium speed, and gradually add more whipping cream until frosting reaches spreading consistency. Cover frosting to prevent drying.