Most 5th wheels do not come with hitches for that purpose. Gulfstream is the only one who can tell you for sure, but I think, he would likely decline to comment for 'liability reasons'.
Be aware that not all states will allow pulling a trailer behind a trailer like that. This is NOT one of those practices that is accepted by all states just because your home state will allow it. This is variously called 'towing doubles' or 'towing triples' depending on who you speak to and you can be required to unhitch if the practice is found unacceptable by a LEO. I don't know if exceptions are ever made for boat trailers or not......
Generally speaking, the western states seem to be more tolerant of this practice than the eastern ones are...... Just an FYI!!!