We went in September. We started our trip with one night in WDW for Mickey's Not So Scary Halloween followed by 6 nights at Hilton Head. I specifically asked the cast member at WDW (Old Key West) if we bought a length of stay cup if it would work at Hilton Head. She had no clue. She went to ask a manager and the manager confirmed it would work. We took the chance and bought one.
To our surprise, it worked perfectly at Hilton Head with no issues. I too was surprised they tied everything in together, but they did! It's the little things in life....