The portal-name must be unique within the war file (or application). The full portlet name is [local|remote].<app-name>.<portlet-name>. The best place to see this is log in as admin, go to the Management Portlet, and click on the Portlets link. Note the names of the portlets as provided in the table.
In the portlet-instances.xml file, the "local.<app-name>" prefix is assumed when referencing the portlet. I believe you can, however, use the full portlet name.