These are some tricky areas of the Drivers Code you are getting in to. Here are the laws as I learned it...
1) Double Towing requires a commerical Drivers License with a double endorsement. (see attached link from CA DMV -- find NOTE on 2nd box right column)
2) Non-Commerical Class A license is required
- for TT with GVWR (not actual weight) >10,000Lbs
- or 5er with GVWR >15,000lbs
http://www.dmv.ca.gov/pubs/dl648/dl648pt2.htm
to clearly answer you first question -- you can't legally tow double with class C license in CA.