Heat grill. In medium bowl, combine butter, lemon peel, lemon juice, marjoram, salt and garlic; mix well. Add shrimp and scallops; toss to coat. Alternately thread shrimp and scallops onto six 12 to 14-inch metal skewers. Reserve butter mixture.
When ready to grill, place kabobs on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium coals; brush shrimp and scallops with butter mixture. Cook 6 to 8 minutes or until shrimp turn pink and scallops turn opaque, turning and brushing frequently with butter mixture. Discard any remaining butter mixture.