In a small bowl, whisk together the warm water, sugar, and yeast. Let sit for about 5 minutes, until foamy and the yeast is activated.
In a separate large bowl, add the potato starch, cornstarch, millet flour, brown rice flour, psyllium husk powder, salt, and xanthan gum. Whisk to combine.
Add the olive oil and egg whites to the yeast mixture and whisk to incorporate.
Add the wet ingredients to the bowl of dry ingredients, then whisk vigorously until completely smooth. The dough will be very wet, almost like batter.
Line a perforated double-baguette pan with parchment paper. (Tip: Use one long piece of parchment paper and fold it in half with a strong crease that straddles the central 'peak' of the double baguette pan.)
Divide the dough evenly between the two halves of the baguette pan, taking care to keep the dough a few inches away from the ends of the pan. It will spread as it rises.
Set in a warm location and let rise for 30 minutes, until approximately doubled in volume. (If possible, cover the loaves during the rise, such as with a large inverted cake pan, but make sure it won't touch the rising loaves.) Meanwhile, preheat your oven to 500º F.
Bake the loaves for 5 minutes at 500º. Then reduce the oven heat to 375º F and bake for an additional 25 minutes.
Remove the loaves from the parchment-lined baguette pan and finish baking for 7 more minutes directly on the oven rack. Then transfer to a wire rack and let cool before slicing.
