For the sauce, combine the four ingredients (plus optional ginger and garlic, if using) in a small saucepan. Bring to a boil over medium heat, whisking until the sugar has dissolved. Remove from heat, set aside, and cool completely.
Preheat the grill to high.
Thread the skewers, starting with the scallions (overlapping and folding them to fit securely on the skewer). Next, alternate the pork belly and chicken pieces to fill a 6-inch skewer. Oil the grill, place the skewers on the grill, and cook 4 to 5 minutes on each side until chicken and pork are cooked through. Serve with remaining sauce.