Whisk together sherry, next 6 ingredients, and, if desired, crushed red pepper. Reserve ½ cup sherry mixture. Pour remaining mixture into a shallow dish or large heavy-duty zip-top plastic bag; add steak. Cover or seal, and chill 2 hours. Add shrimp; cover or seal, and chill 20 minutes.
Remove beef and shrimp from marinade, discarding marinade. Thread beef and shrimp alternately onto 4 (12 inch) skewers.
Grill, covered with grill lid, over medium-high heat (350 to 400 degrees) 10 to 12 minutes or until done. Serve over salad greens with reserved ½ cup sherry mixture for dressing.