In the bowl of a food processor fitted with the steel blade, pulse together brown rice flour, millet flour, tapioca starch, sugar, salt and xanthan gum a few times just to combine. Add the cold butter and pulse again until the butter is the size of small peas, about 30 seconds. Add eggs and continue pulsing until the dough begins to crumble, another 30 seconds or so. Add the cold water all at once and process until the dough comes together. Press the dough together with your fingers. If it holds together, there is no need for more water. If it still feels a bit crumbly, add additional cold water, 1 tbsp at a time, until the dough feels smooth and holds together well.