Cream the shortening and butter together in the bowl of an electric mixer. Add the lemon juice and salt. Gradually add the sugar, 1 cup at a time. Add the meringue powder - the mixture will appear dry. Add the milk and beat until light and fluffy, about 5 to 8 minutes. Keep bowl covered with a damp cloth or plastic wrap.
Yield: frosting and filling for 2 (9-inch) cakes