Expansion is allowed to pass the single authorization for all stores, govened by opencart(multistore). To go to the other store you will be automatically authorized. To be authorized on the one of web-sites you will be automatically authorized on the other web-sites.
Install For opencart 1.5.x
Copy multistore-share-authorize.xml to vqmod/xml/
For opencart 2.x
Go to Extension > Extension Installer and upload the downloaded multistore-share-authorize.ocmod.xml file