In a large saucepan, heat the oil and butter over medium high heat. Add the onion and sausage and cook for about 5 minutes, or until the onion begins to brown.
Add the garlic and cook for another 1-2 minutes, stirring frequently.
Add the tomato, green bell peppers, zucchini, Cajun seasoning, and water. Bring to a boil, then reduce heat to medium-low and simmer, uncovered, for about 15 minutes (the liquid should boil off and the mixture should be thick, the vegetables softened).
Mix in the shrimp and chicken, cover, and simmer for 1-2 more minutes, until heated through.