In a large pot or Dutch oven, heat the vegetable oil over medium heat. Add the chopped onion, garlic, red bell pepper, and green bell pepper. Cook for about 5 minutes, stirring occasionally, until the vegetables are softened.
Add the beef to the pot and cook for about 5 minutes, stirring occasionally, until the beef is browned on all sides.
Add the chicken pieces to the pot and cook for another 5 minutes, stirring occasionally, until the chicken is browned on all sides.
Pour in the 8 cups of water and bring the mixture to a boil. Once boiling, reduce the heat to low and let it simmer for about 45 minutes, or until the beef and chicken are tender.
Add the corn, green plantains, potatoes, and yuca to the pot. Stir well and let the mixture simmer for another 30 minutes, or until the vegetables are tender.
Season the sancocho with salt and pepper to taste. Stir in the chopped cilantro and parsley.
Serve the sancocho hot, making sure to include a piece of beef, chicken, and each vegetable in every serving. Enjoy!