To make the pie crust: in the basin of a food processor, pulse together the flours, sugar, and salt until combined. Add in the butter and pulse until the mixture resembles a coarse meal.
Add in the water—one tablespoon at a time, pulsing after each addition. The mixture is ready when you can grab a handful and squeeze, and it holds together.
Dump the dough out onto a clean surface, and form into a disc. Wrap in plastic wrap and refrigerate for 15 minutes.
To make the filling: Cover the sweet potatoes with water in a medium saucepan over high heat. Bring to a boil and cook until the potatoes are fork tender, about 10 minutes. Drain and mash.
In a blender or the basin of a food processor, combine the mashed sweet potatoes, coconut oil, coconut milk, eggs, nutmeg, cinnamon, vanilla, maple syrup, sugar, lemon juice and salt. Pulse until very smooth. You can also do this in an electric mixer, but the resulting filling will be a bit more chunky.
To assemble the pie: Preheat oven to 350°. Roll the chilled pie crust out to 1/8" thin on a floured surface, form into an ungreased 9" pie plate, crimp edges. Fill with the filling, and bake in the preheated oven for 45-50 minutes, or until a knife inserted an inch from the center comes out clean, and the pie no longer feels very jiggly. To avoid cracks, turn off the oven, crack the oven door, and allow the pie to slowly cool in the oven before removing. Let cool completely before slicing.