In a medium saucepan over medium heat, whisk together 1 1/2 cups of the sugar, cornstarch, and water. Bring to a boil, and cook, stirring constantly, for about 5 minutes or until thickened. Cook a further minute as mixture bubbles.
Add a small amount of the cornstarch mixture to the beaten egg yolks, stirring well to combine. Pour back into saucepan and stir together with remaining cornstarch mixture, continuing to cook over low heat for a further minute.
Remove from heat and stir in butter, lemon juice, lime juice, lemon zest, and lime zest. Set aside.
Preheat the oven to 425 °F.
In a medium bowl using an electric mixer beat egg whites with cream of tartar until soft peaks form. Gradually add remaining 5 Tbs of sugar substitute and continue beating for 2 minutes until firm, glossy peaks form.
Pour the lemon-lime filling into the baked pie crust and smooth over the top. Dollop the meringue over the filling, then spread evenly to cover all the way to the pie crust rim. Make decorative peaks with the back of a spoon.
Bake for 6 to 8 minutes or until the meringue peaks turn golden brown. Cool completely before slicing and serving.
*To make your own pie crust combine 1 cup gluten-free flour blend, 1/4 tsp salt, 1 Tbs sugar and 5 Tbs cold unsalted butter in a food processor.
Pulse to sand-like consistency, then add 3 to 4 TB cold water until dough comes together.
Chill for 20 minutes before rolling out into a 9-inch pie dish.
Bake at 350 °F for 8 minutes or use unbaked according to recipe.