In a large saucepan, melt 1 tablespoon of the butter in the olive oil over moderate heat. Add the onion, garlic, jalapeños and oregano and cook, stirring frequently, until the onion is softened but not browned, about 8 minutes. Add the zucchini mixture and the olives and cook, stirring often, until the mixture is hot and any liquid has evaporated, about 5 minutes. Season with salt. Remove from the heat and let cool for 5 minutes, then stir in the cotija and mozzarella cheeses.