Same server, same db, same drupal installation different domain!

I'm developing a website width domain access and Drupal 7. I've hosted all the website in ONE single webserver with a SINGLE MySQL db, so basically i have a simple drupal installation on a server. The problem is that there are a couple of PRIMARY domains pointing to this server, so if i go to [login to view URL] or to [login to view URL] I'll see the same website(same drupal, same server, same db). My problem is that i need that when a user log in into one domain it must be logged in also in the others, because nowaday if i log in into [login to view URL] i wont be authenticated into the example2.com. It's just a cookie stuff but i have to solve it, i cant use external modules like CAS and other modules that provide this function ONLY on SUB DOMAINS and not on PRIMARY. A good point maybe wold be to use AJAX/CURL for sending the request for setting the cookie for different domains such as [login to view URL] Or also use HTML5 Web Storage for storing the information and then accessing the information from different domains is possible as STACKOVERFLOW does.