How to Make It
Put each chicken in a large resealable plastic bag. In a large measuring cup or bowl, whisk 4 cups of the sake with the soy sauce, mirin, vinegar, chopped ginger and garlic. Divide the marinade between the two bags; press out the air and seal the bags. Turn to coat the chickens thoroughly. Refrigerate the chickens for 24 hours.
In a medium bowl, combine the sake, soy sauce, vinegar, sugar, garlic, scallions and ginger. Season with the hot sesame oil.
Set a steamer rack inside a pot large enough to hold the chickens. Add 6 cups of water to the pot along with the sliced ginger, chopped onion and remaining 2 cups of sake. Bring to a boil. Carefully set the chickens on the rack, cover and steam until cooked through, 60 to 70 minutes. Transfer the chickens to a cutting board or a platter and let rest for 20 minutes.
Carve each chicken into 8 pieces and serve with the dipping sauce.