It is only cross cart if you include the sidebox..

From The Existing Docs.....

If you want to allow the shoppers to have a cross-sites cart (orders products from different websites in the same cart),
You have to display the sidebox called "site_links.php" and you have to add the sites that you want to be link by editing the file
"includes\modules\sideboxes\site_links.php".

In this file, you have to add as many add_site_link(...); as you want of site links.