Heat the oven to 180 °C / 350 °F, work the almond meal, coconut flour, sweetener, cinnamon, salt, lemon zest, and softened butter together in a bowl, then press the dough evenly into a lined baking pan and bake for 10 to 12 minutes. Look for a pale golden surface that still feels soft, rather than a firm, deeply browned cookie.
Toss the walnuts and pistachios with the melted butter, distribute them across the hot crust, and carefully press them into its surface. A lightly embedded, evenly spread nut layer will hold together better than loose piles, but the hot pan and crust require care.
Place the water, allulose, lemon juice, and vanilla in a small saucepan and maintain a simmer over medium heat. The liquid should become thicker, glossy, and sticky before you take it off the heat.
Distribute the warm syrup across the nuts and bake the assembled bars for another 10–12 minutes at 180 °C / 350 °F. Aim for even syrup coverage so no section of topping remains dry.
Leave the baked slab undisturbed until completely cool, then cut it into bars. The soft shortbread becomes firmer as it cools, allowing the crust and topping to stay together during slicing.