I contacted NCWRC in North Carolina to ask about that situation. I was told the spotter could be replaced by mirrors so a two person ski with mirrors could legally tow a single tuber. They mentioned a life jacket but that is a given!
I am sure the coast gaurd would out rank the state level but that is what I have confirmed in North Carolina. If anyone has more detailed info I would be interested.
Here is portion of the email reply I recieved.
".... with motorboats (a pwc is a motorboat). A boat pulling a skier or similar towed device must have one of three things, Mirrors, a spotter or the person being towed must be wearing a life jacket. So to answer your question. If your PWC has mirrors, and the capacity plate allows it, you may tow a person.... "