Preheat oven to 350°F. Stir together cornstarch, lemon juice, and lime juice in a medium nonreactive saucepan until cornstarch dissolves. Stir in 1 1/2 cups water, 1 1/2 cups sugar, butter, 1/8 teaspoon salt, and lemon zest. Bring to a boil over medium-low, stirring constantly until thickened, about 15 minutes. Remove from heat.

Whisk egg yolks in a large bowl. Gradually whisk in hot lemon-lime mixture. Pour mixture into pastry shell. Press a piece of plastic wrap directly onto surface, and let cool 30 minutes.

Combine egg whites and remaining 1/8 teaspoon salt in bowl of a heavy-duty mixer. Beat on medium speed until frothy, about 1 minute. Gradually add cream of tartar and remaining 1/2 cup sugar, beating until mixture forms stiff, glossy peaks, about 4 minutes.

Gently dollop meringue over warm lemon-lime filling. Spread meringue to edges with a spoon. Broil in preheated oven until meringue is golden brown, about 1 to 2 minutes. Transfer to a wire rack, and let cool to room temperature, about 1 hour. Transfer to refrigerator until well chilled, about 3 hours.