Preheat oven to 425 F. Line a 9-inch pie plate with 1 layer of pastry; set aside.
Combine 1 cup of water and the salt in a large saucepan. Add sweet potatoes; cover and bring to a boil over high heat. Reduce heat and simmer until sweet potatoes are just barely tender when pierced, about 5 minutes. Drain slices, rinse with cold water until cool to touch. Drain slices well; transfer them to a large mixing bowl.
To sweet potatoes, add sugars, lemon juice, flour and pumpkin pie spice. Toss gently with a spatula until slices are evenly coated. Spoon sweet potatoes into pastry-lined pie plate; dot top of potatoes with butter. Place top pastry over filling and crimp top and bottom pastries together to make decorative edge. Cut 3 to 4 slits in the top to allow steam to escape.
Bake in oven until crust is browned and sweet potatoes are tender when pierced, about 50 minutes.
Cool pie on wire rack at least 1 hour. Serve warm or cool.