Heat oil in a large saucepan over medium heat. Add onion; cook, stirring constantly, for 3 minutes. Add garlic, thyme, fennel seed, salt, pepper and saffron; cook for 20 seconds.
Stir in tomatoes and broth. Bring to a simmer. Cover, reduce heat and simmer for 2 minutes.
Increase heat to medium, stir in scallops and cook, stirring occasionally, for 2 minutes. Add shrimp and cook, stirring occasionally, for 2 minutes more. Stir in couscous. Cover, remove from heat and let stand for 5 minutes; fluff.