Yes, we track each buyer's cart contents/settings by setting a cookie in their browser, which by necessity must be set for a specific domain. Since our standard overlay-style cart works as part of your own page, that cart has to set its cookies with the domain in your page's URL, and so carts on different domains cannot share contents; thus, items added to one domain's cart won't show up in the other and vice-versa.
However, there is one way to work around this, if you don't mind giving up the overlay-style cart presentation. If you use the non-JavaScript version of our cart button code, that will pop-up a separate window/tab to display the cart, and since this pop-up cart has its own page URL on the e-junkie.com domain, the cart cookie gets set for that domain, and it won't matter what domains your pages are using. As long as the items are all set up as products in the same E-junkie account's Seller Admin, they can be added to a buyer's pop-up cart from pages on different domains.