Whisk 2 tablespoons oil and lime juice in a large bowl. Add fish and sprinkle generously with salt and pepper; stir to coat. Let stand 15 minutes.
Heat remaining 2 tablespoons oil in a large pot over medium heat. Add onion, bell peppers, garlic and crushed red pepper; saute 5 minutes. Mix in tomatoes, coconut milk, half of the cilantro and half of the green onions. Add shrimp and fish with marinade. Simmer until shrimp and fish are just opaque in center, about 5 minutes. Season sew with salt and pepper. Transfer to a bowl and sprinkle with remaining cilantro and green onions.