Soak noodles in hot (not boiling) water to cover until soft, 5 to 10 minutes; drain.
Mix catsup, sugar, 1/2 to 1 teaspoon chili flakes (to taste), and 1/4 teaspoon salt or 1 tablespoon fish sauce.
Place a nonstick 12-inch frying pan or 14-inch wok over high heat. When pan is hot, add oil, garlic, chicken, and shrimp. Stir-fry until shrimp begins to turn pink, about 1 1/2 minutes.
Reduce heat to medium-high. Add eggs and drained noodles; stir until mixed. Add catsup mixture and stir until noodles are tender to bite, about 2 minutes.
Add onions, 2 cups bean sprouts, and salt or fish sauce to taste. Mix.
Spoon onto plates. Sprinkle with peanuts and cilantro sprigs. Offer remaining bean sprouts, cabbage, carrot, lime, and chili to add to taste.