Preheat oven to 450*.Line an unpricked crust with a double thickness of foil.Fill with pie weights, dried beans or uncooked rice.
Bake until bottom is lightly browned, 8-10 minutes.Remove foil and weights; bake until golden brown, 5-8 minutes longer.Cool on a wire rack.Reduce oven setting to 350*.
Meanwhile, in a saucepan, mix sugar, cornstarch and salt.Whisk in water and orange juice.Cook and stir over medium-high heat until thickened and bubbly.Reduce heat to low; cook and stir 2 minutes longer (mixture will be thick).Remove from heat.
In a small bowl, whisk a small amount of hot mixture into beaten egg yolks; return all to pan, whisking constantly.Bring to a gentle boil; cook and stir 2 minutes.Remove from heat.Gently stir in lemon juice, butter and lemon and orange zests.
For meringue, in a large bowl, beat egg whites with vanilla and cream of tartar on medium speed until foamy.Gradually add sugar, 1 tablespoon at a time, beating on high after each addition until sugar is dissolved.
Continue beating until soft glossy peaks form.
Transfer hot filling to crust.Spread meringue over filling, scaling to edge of crust; swirl top with the back of a spoon.
Bake until meringue is golden brown, 13-16 minutes.Cool on a wire rack 1 hour.Chill 3 hours before serving.Refrigerate leftovers.
