Buttermilk Brined Chicken with Cress and Bread Salad

Name: Buttermilk Brined Chicken with Cress and Bread Salad
Ingredients: 2 1/2 lemons; 2 quarts buttermilk; 4 garlic cloves, crushed; 2 tablespoons kosher salt plus more; 2 3 1/2-4-pound chickens; 1 bunch fresh marjoram; 1 /4 cup (1/2 stick) unsalted butter, room temperature; Freshly ground black pepper; 1 small shallot, finely chopped; 1 /2 cup olive oil; 1 /3 cup Champagne or white wine vinegar; 1 loaf country-style bread or ciabatta, crust removed, torn into 1 1/2-inch pieces (about 8 cups); 2 bunches watercress, thick stems trimmed (about 8 cups)