Just FYI, I was purchasing a one-day Pass for a friend of my daughter's for DCA on Friday and the CM asked if I was a So Cal resident. Since I was, she told me they are now running a special deal (as yet unadvertised, or advertised very little) that allows So Cal residents to get a one-day HOPPER pass that is good for BOTH parks at the same price as a regular pass! This pass CAN be used on the same day or ANY TIME up to 30 days after the initial purchase. Great deal! 
It is ONLY available at the ticket gates, not in advance.

It is ONLY available at the ticket gates, not in advance.