Preheat the oven to 450 degrees F. Lightly oil a baking sheet large enough to hold the two chickens
Stuff each chicken with 3 garlic cloves, 2 sprigs, rosemary, 2 sprigs thyme, and 1/4 teaspoon crushed peppercorns. Truss the chickens with kitchen twine. In a small bowl, combine the mustard, chopped thyme, chopped tarragon, and parsley. Using a pastry brush, brush the mustard mixture all over the chickens. Place the chickens, breast sides up, on the prepared baking sheet.
Divide the dough in half. According to the directions in the Salt Crust Dough recipe, roll out each portion as thinly as possible. Wrap each chicken completely with dough.
Bake until the dough is browned and rock-hard, about 50 minutes. Remove from the oven and let stand for 10 minutes. Using a hammer or the back of a heavy knife, crack open the crust and remove the chicken, brushing off any clinging crust with a clean kitchen towel. Cut each chicken in half lengthwise. Serve immediately on warmed plates