Whisk buttermilk, garlic, and 2 tablespoons salt in a medium bowl. Divide buttermilk mixture and lemon slices between 2 large resealable plastic bags.
Place 1 chicken in each bag and seal, pressing out any excess air. Refrigerate, turning occasionally, at least 8 hours and up to 1 day.
Preheat oven to 425°F.
Remove chickens from marinade, rinse with cold water, and pat dry. Thinly slice half of a lemon; cut remaining whole lemon in half.
Place chickens on a wire rack set inside a large rimmed baking sheet; stuff each with 1/2 bunch of marjoram and a lemon half. Working from neck end of chicken, gently loosen skin from chicken breasts.
Place several lemon slices under skin of each breast. Rub chickens with butter; season with salt and pepper. Tie legs with kitchen twine and tuck wing tips under chicken.
Roast chickens until they begin to brown, 30-35 minutes. Reduce oven temperature to 350°F and roast until chickens are cooked through and a thermometer inserted into the thickest part of the thigh registers 165°F, 20-25 minutes longer.
Transfer chickens to a platter, tent with foil, and let rest while preparing bread salad. Keep oven on.
Carefully pour chicken roasting juices from baking sheet into a large bowl (set baking sheet aside).
Whisk shallot, oil, and vinegar into juices in bowl; season vinaigrette with salt and pepper.
Pour half of vinaigrette into a small bowl; set aside.
Add bread to vinaigrette in large bowl and toss, pressing gently to help bread absorb vinaigrette.
Transfer bread to reserved baking sheet and toast, tossing occasionally, until bread is golden brown but still soft in the center, 10-15 minutes (set bowl aside).
Place warm bread in reserved bowl.
Add watercress and reserved vinaigrette; season with salt and pepper and toss to combine. Arrange salad on a platter and place chickens on top.
