Mix together the first amount of sugar and water. Bring to a boil and remove from heat. Sift cornstarch and second amount of sugar together and whisk into the egg yolks with zest. Mix well. Add a small amount of the hot sugar mixture to the eggs while whisking constantly, to gradually warm up the egg mixture without scrambling the eggs (the process is called tempering). Once the eggs are warmed up, whisk the mixture into the pot of hot sugar water and put it back on medium-high heat. Stir constantly and bring to a boil (it will thicken considerably). Remove from heat and stir in vanilla, butter and 1 cup of reserved juice. Set aside.
For the meringue: Whip the whites on medium heat with cream of tartar. When whites start to get foamy and thicken, gradually add sugar while continuing to whisk. Once all sugar is incorporated, turn mixer to high and whip until the meringue becomes shiny and holds a stiff peak (don't allow them to become dull and spongy).
Assembly: Divide the filling and pour evenly into the two prebaked shells; allow filling to cool entirely. Top with meringue, and style it as desired into attractive peaks. Using a torch, wave the flame back and forth over the meringue to cook and set it. It should be an attractive golden brown color (alternatively, you could briefly place the pie in an oven set to 450F until you achieve the same colour).
Yield: 2 pies