Buttermilk Brined Chicken with Cress and Bread Salad

INGREDIENTS

• 2 1/2 lemons
• 2 buttermilk
• 4 garlic cloves, crushed
• 2 kosher salt plus more
• 2 3 1/2-4-pound chickens
• 1 bunch fresh marjoram
• 1/4 (1/2 stick) unsalted butter, room temperature
• Freshly ground black pepper
• 1 small shallot, finely chopped
• 1/2 olive oil
• 1/3 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)