1. Preheat oven to 300°. Chop pistachios in a food processor a few seconds until finely chopped. Add walnuts, 2/3 cup sugar, and cinnamon, and process until the texture of coarse meal; pulse out any lumps. Set aside.
2. Lightly fit 1 phyllo sheet in the bottom of a lightly greased 13- x 9-inch baking dish; trim phyllo to fit, if necessary. Brush well with melted butter. Fit 7 more phyllo sheets in dish, brushing each layer well with melted butter.
3. Scatter a generous 1/3 cup nut mixture over phyllo. Add 2 more phyllo sheets, brushing each with melted butter, then another 1/3 cup nut mixture. Continue layering phyllo and nut mixture, brushing each sheet with butter, until all nut mixture is used. Top with remaining phyllo sheets, brushing each with melted butter. Place baklava in freezer 10 minutes. (This makes it easier to cut.)
4. Slice baklava in a 2-inch-wide, diamond-shaped pattern. (At this point, baklava may be frozen: Wrap pan with heavy-duty aluminum foil, label and date, and freeze.)
5. Set baking dish on middle oven rack, and place a shallow pan of water on rack underneath. Bake at 300° for 2 hours and 10 minutes or until golden brown. (Bake frozen baklava, uncovered, in same way at 300° for 2 1/2 to 3 hours or until golden brown.)
6. Combine remaining 2/3 cup sugar, 2 cups water, honey, and lemon juice in a large, heavy, nonstick saucepan. Bring to a boil over high heat, reduce heat to low, and simmer, uncovered, 20 minutes. Transfer to a 1-quart glass measuring cup, and cool to room temperature. Drizzle syrup evenly over baklava. Cool to room temperature, cover loosely with foil, and let stand overnight before serving.