- subdomain.domain.com needs to be entered in Magento’s Admin->System->Configuration->Web->Cookie Domain, to make session cookies available to other subdomains.
- subdomain.domain.com needs to be entered in Magento’s app/code/core/Mage/Core/Model/Config.php file, to make session cookies available to other subdomains.
- Allowing access to session cookies from other subdomains is not part of Magento’s core functionality, but it can be achieved using extensions.
- It is not possible to access a Magento site’s cookies on a different subdomain.
Тупой индус наверняка имел в виду вариант 1.