Peel and devein shrimp. Place shrimp and scallops in a small heavy-duty, zip-top plastic bag. Combine shallots and next 8 ingredients; pour over shrimp and scallops. Seal bag, and shake until shrimp and scallops are well coated. Marinate in refrigerator 1 hour, turning bag occasionally.
Remove shrimp and scallops from marinade, reserving marinade. Thread shrimp and scallops onto 2 (12-inch) skewers. Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place kabobs on rack; grill, uncovered, 3 minutes on each side or until scallops are opaque, basting frequently with marinade. Sprinkle with parsley; garnish with lemon wedges, if desired. Serve immediately.