1, Remove and discard any loose pieces of fat from the chicken parts. Wielding a heavy cleaver designed for chopping bones, whack the bones to break them partway or all the way through, making the cuts at 1- to 2-inch intervals, depending on the size of the part. This exposes the marrow, which enriches the broth.
2, Put the chicken, pork bones, and water in a stockpot (about 12-quart capacity) and bring to a boil over high heat. Lower the heat to a gentle simmer and then use a ladle or large, shallow spoon to skim off any scum that rises to the top. Add the onion, ginger, rock sugar, and salt. Simmer gently, uncovered, for 2 hours.
3, Turn off the heat and let the broth sit undisturbed for 30 minutes to allow the impurities to settle and congeal. Position a fine-mesh sieve (or a coarse-mesh sieve lined with cheesecloth) over a pot and gently ladle the broth into the sieve. Discard the solids. Use a ladle to skim as much fat from the top of the broth as you like. (To make this task easier, you can cool the broth, refrigerate overnight, and then lift off the solidified fat.) The broth is ready to use. Or, store it in a tightly covered container in the refrigerator for up to 1 week or in the freezer for up to 3 months