Heat olive oil in a wok over medium-high heat. Saute onion and garlic until onion is translucent, about 5 minutes. Add pork and chicken; cook and stir until lightly browned, about 5 minutes. Add soy sauce and sugar. Mix well; season with salt and pepper. Simmer until mixture is nearly dry and pork and chicken are no longer pink, about 10 minutes more. Let cool before filling the siopao dough.