Preheat the oven to 325˚Line an 8 ½ x 4 ½-inch loaf pan with parchment paper (leave extra hanging over the sides for easy removal).
In a food processor, combine the almond flour, coconut flour, baking powder, Swerve, arrowroot powder, and salt. Pulse until combined.
Slowly add the melted butter. Pulse until crumbly, scraping down the sides as needed.
Using an electric mixer, beat the egg whites and cream of tartar until stiff peaks form.
Add half of the egg whites to the food processor. Pulse a few times until just combined (do not over-mix).
Carefully transfer the mixture from the food processor into the bowl with the remaining egg whites. Gently fold until combined (do not stir).
Pour the batter into the loaf pan and smooth the top.
Bake for about 40 minutes or until the top is golden brown.
Create a tent with aluminum foil, and bake for an additional 30 to 45 minutes until the top is firm and the internal temperature is 200˚F.
Allow to cool completely before removing from the pan and slicing