Rinse chickens, and pat dry with paper towels. Place 4 thyme sprigs in cavity of each bird. Rub each chicken with 2 tablespoons softened butter. Squeeze a lemon half over each chicken. Sprinkle with salt and pepper.
Tie ends of legs together with string; tuck wing tips under. Place chickens, breast side up, in a well-greased shallow roasting pan.
Toss garlic with olive oil in a bowl. Scatter garlic cloves around chickens, snuggling them close to chickens.
Bake at 375° for 1 hour and 45 minutes or until a meat thermometer inserted into thigh registers 180°.
Remove chickens and garlic to a serving platter. Garnish, if desired.