Preheat oven to 400°F. Stir salt and 5 egg whites in large bowl, adding more egg whites as needed to form grainy paste. Press ¼- inch layer of salt mixture (large enough to hold both fish) onto large rimmed baking sheet. Stuff cavity of each whole fish with half of herb sprigs and lemon slices. Place fish atop salt mixture on baking sheet. Pack remaining salt mixture over fish to enclose completely. Roast until thermometer inserted into thickest part of fish registers 135°F, about 20 minutes. Let stand 10 minutes.
Using back of large spoon, gently crack open salt crust on fish. Lift and discard salt layer. Pull skin from top of 1 fish. Carefully lift top fillet from bones and transfer to plate. Lift and discard bones. Gently lift second fillet from skin and transfer to second plate. Repeat with second fish for a total of 4 plates.