Would be interesting to hear an opinon of Japanese, but since no one is here yet I'll give my 2 cents.
From what I know I would say that かっこいい is mainly used to describe something or someone looking cool. A stylish girl, sharp looking man, well tuned car and etc. Also it is used to describe a person acting cool. A gardener who normally isn't considered cool can do his job with presice, fast and efficient movements, so it is also kakkoii. So, in other words, if you want to express your... admiration, I guess, you use this word.
In case of 凄い, it is broader and used to express amazement by something, positive or negative one. It depends on the context and tone of the speaker. If used positively, I don't think it is so different from English "cool". If used negatively, it can be translated as "ridiculous", "strange" and etc.
So yes, in some cases they can be interchanged.