One day before serving: In medium sized bowl, with electric mixer at high speed, beat whites and cream of tartar until soft peaks form. Gradually add sugar, beating until sugar dissolves and stiff peaks form. Heat oven to 225F and line two baking sheets with greased aluminum foil. Draw 3, 8 inch circles. Using a pastry bag, with star tip, pipe a border, inside the edge of each circle, on baking sheets. Do not extend beyond the circle. On one circle, pipe a second border inside the first. Divide remaining meringue into centers of circles. With small spatula, spread meringue to fill centers of circles and to touch shell borders. Bake meringues 3-4 hours or until crisp. Turn off oven. Allow meringues to stand in oven several hours or overnight. An hour before serving, place meringue layer with single border on serving plate. Top with one third of chilled rhubarb sauce. Repeat with second single-border meringue and one third of sauce. Top with double-border meringue and remaining sauce. Cover torte carefully and refrigerate until read to serve.