Preheat oven to 425°. Move rack to middle of oven. Stuff cavity of each fish with herb sprigs and slices of lemon and garlic, dividing evenly. Coat fish with olive oil.
In a very large bowl, combine salt with egg whites and 2 cups water. Mix with your hands, then add in another 2 cups water, mi, until the salt feels like moist sand (you may need a little more water).
Line a large rimmed baking sheet with 1/2 in. of the salt mixture. Arrange the two fish nose to tail on the salt, leaving at least 1 1/2 in. space between them. Cover each with an even 1/2-in. layer of salt mixture, patting to adhere (no part of fish should remain exposed) and leaving a crevice between the fish so they cook evenly. Bake 30 minutes, or until salt crust is very pale gold and hard to the touch, like ceramic.
Meanwhile, make sauce: Whisk all ingredients in a bowl. Set aside for at least 30 minutes to allow the flavors to marry (but not more than 2 hours ahead or the herbs will lose their fresh taste).
Working with one fish at a time, tap hard salt crust with a small hammer or meat mallet to crack it. Lift off chunks of crust and brush any remaining salt off fish (a pastry brush is useful here).
With a large fork, carefully peel back skin, starting at spine and working toward belly. Lift off chunks of meat with fork and transfer to a platter. Lift out skeleton with the head attached and discard. With fork, transfer the rest of the meat to the platter.
Repeat dismantling of salt crust and filleting with remaining fish, adding meat to the platter. Serve with dressing spooned over top and more served on the side.
Note: Nutritional analysis is per 1/2 fillet with 1 Tbsp. sauce.