One thing that you should know is that these two terms are known to be synonymous with each other. There are some technical differences, though, that will differentiate one from the other. First of all, CECA means the Comprehensive Economic Cooperation Agreement. CEPA means the Comprehensive Economic Partnership Agreement.
When CECA is used, this means that there is some sort of connection between the countries that are trying to work together. For CEPA, the two countries do not just have a random relationship. They are meant to work well together so that they can have a partnership with each other. Take note that the use of these two will require negotiation between the countries that are involved.