Connecting two wordpress sites.

Hey,

I have two wordpress websites on different domains and using different database.
I want users from site 1 to automatically login to site 2. But Site 2 users won’t be allowed on site 1.

I was searching about it. And I found couple of possible solution.

define('CUSTOM_USER_TABLE', 'wp_users');
define('CUSTOM_USERMETA_TABLE', 'wp_usermeta');

I was wondering “wordpress.com single sign on” works in a similar way, If that can do the trick.
I would like to connect both of them using Single sign on or any other method.

Please suggest me a way or any plugin which can make it happen.

Thanks a lot!

Looking forward to replies.