Preheat the oven to 350 degrees F. Place a piece of parchment paper or a Silpat on a baking sheet.
Sift each of the flours through a fine-mesh sieve into a large bowl. Sift the almond flour particularly well, pushing the last of it through the sieve with the bottom of a heavy glass. Toss in the baking powder, xanthan gum, and salt. Stir all the ingredients together. Sift the dry ingredients through the sieve again, which will make them one flour.
Place the softened butter in your stand mixer, with the paddle attachment. Start the mixer, beating the butter, and then pour in the sugar. Cream them together until they are smooth, about 3 minutes. Plop in the eggs, one at a time, and continue beating, until the mixture is lovely and fluffy. Toss in the lemon zest and mix. Reduce the mixer speed to as low as it goes.
Sift the dry ingredients into the creamed butter and sugar, 1/3 cup at a time, allowing the mixer to run in between batches. When you have finished, the dough will be soft and gathering around the paddle attachment.
Grab half the dough with your hands and shape it into a long, low log on the baking sheet. Aim for a rough approximation of a log, a little squat and comfortingly uneven. Form a log with the rest of the dough and place it on the baking sheet.
Slide the baking sheet into the oven, on the middle rack. Bake the biscotti until the biscotti logs are golden and somewhat firm, but still somewhat soft, about 15 minutes. Take the baking sheet out of the oven and allow the biscotti to cool for 10 minutes. Move them to a cooling rack and turn your back on them for at least 30 minutes.
When the biscotti are entirely cool to the touch, move the biscotti logs back to the baking sheet. Take a large serrated knife and make slices on the bias, all down the logs. Be gentle. This is a delicate operation. Stand the biscotti slices on their squat bottoms, on the baking sheet. Slide the baking sheet back into the oven. Bake the biscotti until they are unequivocally browned and crunchy, about 15 minutes.