Stir together first 4 ingredients. Press mixture evenly into a 9-inch tart pan with removable bottom; set aside.
Whisk together sweetened condensed milk, orange juice concentrate, lemon juice, and egg yolks until blended.
Beat egg whites at medium speed with an electric mixer until stiff peaks form; fold into condensed milk mixture. Pour into prepared crust.
Bake at 325° for 20 to 25 minutes or just until filling is set. Remove to a wire rack, and let cool completely. Cover and chill at least 4 hours. Remove tart from pan, and place on a serving dish.
Beat whipping cream and granulated sugar at medium speed with an electric mixer until stiff peaks form. Dollop around edges of tart; garnish, if desired.