Preheat the oven to 325 degrees F. Spray a 17-by-12-inch rimmed baking sheet with nonstick cooking spray.
Unroll the pie crusts and place them in the prepared baking sheet. Line them up so they meet in the center of the baking sheet and overlap a bit. Cut the crusts to fit together in a rectangle slightly larger than the baking sheet, pinching the center seams together. Crimp the dough around the edges of the baking sheet and cut off any excess dough around the edges. Chill while preparing the filling.
In a large bowl, whisk together the brown sugar, granulated sugar and eggs until incorporated. Stir in the butter, milk, flour, lemon zest and juice, vanilla and 2 cups chopped pecans until combined.
Pour the mixture into the unbaked pie shell. Arrange the pecan halves on top of the pie. Bake until the crust is golden brown and the filling is set but still a little loose in the center (it will set as it cools), about 50 minutes. Serve warm or at room temperature.