Prepare grill or heat broiler.
Stir together half of yogurt and 1/4 cup milk in large bowl. Add chicken; marinate 10 minutes.
Combine chili powder, cumin, 1/4 teaspoon each salt and pepper.
Bring chicken broth and water to boiling in medium-size saucepan. Remove from heat. Add couscous; cover and let stand 5 minutes. Transfer couscous to large serving platter; let cool to room temperature.
Stir together remaining yogurt, sour cream, remaining milk, dill, green onion, garlic and remaining salt and pepper. Add cucumber. Spoon cucumber salad onto cooled couscous. Refrigerate.
Remove chicken from marinade; remove most of yogurt from chicken. Sprinkle top with half the spice mixture; rub in.
Grill or broil 5 inches from heat 4 to 5 minutes. Turn chicken; sprinkle with remaining spice mixture. Grill or broil 4 minutes until cooked. Let cool 5 minutes. Slice diagonally; arrange on couscous salad. Garnish with dill.