1. Heat grill to high (450° to 550°). In a small bowl, combine harissa, 2 tbsp. each oil and lemon juice, 2 tsp. zest, the cumin, and 1/2 tsp. salt. Put shrimp in a medium bowl, toss with half the sauce (reserve the rest), and let stand 10 minutes. Thread shrimp onto metal skewers.
2. Prepare couscous as package directs, adding remaining 1 tsp. zest. Fluff with a fork and gently stir in 1 tbsp. oil and remaining 1 tbsp. lemon juice and 1/2 tsp. salt. Set aside.
3. Brush tomatoes with remaining 1 tbsp. oil. Grill shrimp and tomatoes, covered, turning once, until shrimp is opaque and tomatoes are softened, about 4 minutes. Serve with couscous plus a sprinkle of mint, more salt to taste, and remaining sauce.
*Find this North African chile-and-spice paste in the international section of supermarkets. We like Dea brand, which is medium-hot.