In large bowl, cream shortening and butter with electric mixer until light and fluffy. Add lemon juice and zest; beat well. Gradually add sugar, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been mixed in, if icing is dry, add additional juice or milk and beat at medium speed until light and fluffy.
Keep icing covered with a damp cloth until ready to use. For best results, keep icing bowl in refrigerator when not in use. Re-whip before using.