Been carrying this for awhile, as we planned to travel there with our trailer/swivel wheel.
California codeDriver Licensing For Double Trailers - Nonresidents
12502. (a) The following persons may operate a motor vehicle in this state without obtaining a driver's license under this code: (1) A nonresident over the age of 18 years having in his or her immediate possession a valid driver's license issued by a foreign jurisdiction of which he or she is a resident, ....
(b) Any person entitled to the exemption contained in subdivision (a), while operating, within this state, a commercial vehicle, as defined in subdivision (b) of Section 15210, shall have in his or her possession a current medical certificate of a type described in subdivision (c) of Section 12804.9, which has been issued within two years of the date of operation of that vehicle.
15210. (b) (1) "Commercial motor vehicle" means any vehicle or combination of vehicles which requires a class A or class B license, or a class C license with an endorsement issued pursuant to paragraph (4) of subdivision (a) of Section 15278.
In summary, nonresidents may tow two trailers with a non-commercial license IF their base state allows it. IF the vehicle is commercial, the driver would also need a valid medical certificate per 12502 CVC.