To get 2 clutches from a single female in the same year, you start to try to breed her again after her first post-laying shed. This is about 2 weeks after she lays her first clutch. Only attempt this if the female is of a GOOD size and has not dropped below her pre-first breeding weight. And definately do not double-clutch a first time breeder. The only reason I double-clutch my 5 yr old or older females is that, through experience, 90% of them have , and most likely will again, lay a second clutch whether I like it or not, so I figure that they might as well be fertile. And I still always follow my above mentioned conditions. If a female doesn't measure up, she lays slugs if she is going to lay.
The coolong off period refers to brumation (hibernation), which generally is done during the winter months, usually starting late Nov to Dec. Corns are fed a last meal, allowed to digest and pass the remains, then are gradually cooled down into about the 60's F at night with daytime highs into the upper 70's F, with an occasional allowed jump into the low 80"s (sometimes unavoidable here in Florida) Low's can go into the 50's F, but should never be for long periods. After about 2 months, temps are gradually raised back to normal, then feeding is resumed, starting with small prey. Breeding will start after the first post-brumation sheds.
For further, more detailed information, I would highly suggest that you purchase and read Kathy Love's Corn Snake Manual. It's $10 well spent.