eCart is a developer tool that leverages Dreamweaver to add additional functionality.
It is certainly possible to learn both Dreamweaver and eCart at the same time, but that really is just adding an extra layer of complication.
that's not to say that one must be a master of Dreamweaver, but a working knowledge of how to create a basic site would be very helpful before trying to move on to adding eCommerce to the site.
a basic understanding of defining a dreamweaver site for dynamic development, creating a page design, and adding dynamic content is a good idea before trying to work with eCart.