1. Preheat oven to 425°. Unroll piecrusts, and stack on a lightly floured surface. Roll piecrusts into a 14- x 10-inch oval. Fit piecrust into a lightly greased 12- x 8-inch rectangular tart pan with removable bottom; press into fluted edges. Trim off excess crust. Line piecrust with aluminum foil or parchment paper, and fill with pie weights or dried beans. Place pan on a baking sheet.
2. Bake at 425° for 10 minutes. Remove weights and foil, and bake 7 to 8 more minutes or until browned. Cool completely on baking sheet on a wire rack (about 15 minutes). Reduce oven temperature to 300°.
3. Meanwhile, grate zest from grapefruit to equal 1 tsp. Peel grapefruit; section over a bowl to catch juice, reserving 1/3 cup juice and segments separately. Reserve any remaining juice for another use. Whisk zest and salt into 1/3 cup juice.
4. Whisk together egg whites and yolks in a large bowl until creamy and light in color; whisk in cream and butter.
5. Stir together sugar, flour, and cornmeal in a medium bowl. Gradually whisk sugar mixture into egg mixture, one-third at a time, until blended. Whisk in grapefruit juice mixture. Spoon filling into cooled tart shell.
6. Bake at 300° for 20 minutes; remove from oven, and arrange grapefruit sections on tart. Bake 30 to 35 more minutes or until fruit is browned and filling is set. Cool tart on a wire rack 1 hour before serving.