In a large pot, add the chicken bones and pour in 21 cups (5 liters) of water. Bring to a boil and skim off any foam. Lower the heat and simmer for 40 minutes. After 40 minutes, add the two whole chickens and cook for another 20 minutes until the chicken is cooked through, then remove the chicken.

While the broth is cooking, roast the ginger, shallots, star anise, and sawtooth herb until fragrant, then add them to the pot along with the chicken. Season the broth according to the recipe to achieve the perfect chicken pho flavor.

Note: During cooking, the broth may reduce and fall short of 21 cups (5 liters). Add more water as needed to maintain the correct amount, otherwise the broth may become too salty.