with a single login , i need the user to login into one site and he is also able to access the other site .,without logging in again
Good on ya !
1) If both sites are in the same base domain (site1.xxx.com and site2.xxx.com) you can store an authentication token as a cookie and pass it to between sites. If the sites are in different domains (site1.xxx.com and site2.yyy.com), this won't work.
2) If you can control the link that the user follows to move to the next site, you can encode an authentication token into the URL for that link.
If neither of those options work, I suggest you look at a single-signon software package, because it would probably be cheaper to buy such a package than try to build something yourself.
you'd have to work with a cookie... that recognizes the user once he logs in. i agree with paul strack's suggestions