In a large, nonreactive bowl, combine ½ teaspoon of the lime zest, ½ teaspoon of the lemon zest, the lime and lemon juice, and 2 tablespoons of the chopped cilantro. Add the fish and shrimp and refrigerate, covered, for 30 minutes.
Melt the coconut oil in a medium soup pot over medium heat. Add the onion and sauté for 10 minutes, or until translucent. Add the garlic and sauté for 30 seconds more.
Add the tomatoes, coconut milk, fish sauce, and cayenne pepper. Simmer for 10 minutes, covered.
Add the fish and shrimp along with the marinade. Bring to a simmer and cook another 6 to 8 minutes, or until the fish starts to flake and the shrimp is cooked through.
Serve garnished with the remaining cilantro and citrus zest.
Make sure you: zest the lemon and lime before juicing them.