Preheat oven to 350°F and line a 9x5-inch loaf pan with parchment paper.
In a large bowl, whisk together almond flour, coconut flour, sweetener, baking powder, and salt.
In a separate bowl, beat eggs, then add melted butter, almond milk, and vanilla extract and mix well.
Pour wet ingredients into dry ingredients and stir until a thick batter forms with no lumps.
Pour half the batter into the prepared loaf pan and sprinkle with half of the cinnamon-sweetener mixture.
Add remaining batter on top and sprinkle with remaining cinnamon mixture, then use a knife to swirl the mixture through the batter in a zigzag motion.
Bake for 50 minutes or until a toothpick inserted in the center comes out clean.
Allow to cool in the pan for 15 minutes, then transfer to a wire rack to cool completely before slicing.