I have heard that CA requires a Class A CDL with Doubles Endorsement for towing two trailers behind anything, recreational or commercial.
The reason given for the CDL is that it is the only license in CA that can carry a Doubles Endorsement.
I have heard that in CA the first trailer must be a fifth wheel hitch, and the second trailer must be a watercraft. I have also heard that this WAS true, but has been changed.
I do not know if this is true. I have not investigated the issue, because I will never tow doubles in CA, nor will I ever be a resident of CA.