In a small bowl cream shortening and margarine together until well mixed.
Add the icing sugar in 1/2 cup additions. After each addition add a splash of lemon juice and beat well (this is easiest with hand held mixer). Add vanilla and beat for another 3-5 minutes until you have a smooth creamy and fluffy frosting.
Wrap tightly in plastic and refrigerate until needed. Pipe or spread over cakes and refrigerate to set.