Put chicken, 1/2 cup of yogurt and spice mix in a resealable plastic food storage bag. Seal bag and turn to mix and coat. Place bag on a plate and refrigerate on lowest shelf for at least 1 hour or up to 8 hours.
To make sauce, stir together remaining 3/4 cup of yogurt, cucumber, mint and salt in a small bowl until blended; refrigerate until ready to serve.
Preheat grill. Remove chicken from marinade; discard marinade. Thread chicken and onion wedges onto 8 metal skewers, using 4 pieces of chicken and 2 pieces of onion per skewer. (If using wooden skewers soak them in water 30 minutes before use to prevent burning.) Coat chicken and onions with cooking spray.
Grill skewers, turning often, until chicken is no longer pink in center, about 7 to 9 minutes. Serve skewers over couscous with sauce on the side. Yields 2 skewers, 1/2 cup of couscous and about 1/4 cup of sauce per serving.