Roast Chickens With Shallots and Grapes

Name: Roast Chickens With Shallots and Grapes
Ingredients: 2 4-lb. whole chickens, neck and giblets removed; 5 1/2 tsp. kosher salt; 1/2 cup (1 stick) unsalted butter, softened; 2 Tbsp. finely chopped fresh herbs (such as flat-leaf parsley, thyme, and rosemary), plus more for garnish; 2 tsp. poultry seasoning; 1/2 tsp. freshly ground black pepper; 4 large cloves garlic, grated on a Microplane grater (about 1 Tbsp.); 8 large shallots, peeled and halved lengthwise; 4 sprigs rosemary; 1 cup lower-sodium chicken broth, divided; 1 1/2 lb. red seedless grapes, in small bunches on the stem (about 6 cups); 1 Tbsp. cornstarch; 2 tsp. Sherry or red wine vinegar