How to sync users profiles and user tables in 2 drupal sites I have a website with 14K registered users. There is new section of that website which has been developed as a separate Drupal installation. The users in the old website should be able to login with their username and password to the new site.

My questions are :


is there a way to connect new site "user registration/login/forget
password" features with the old one? 
Should i sync the databases all
the time? Or when the new user registers, his information is added
in to new and old site ( and Vice Versa)


What is the best practice in such cases.

thanks :)
There are many ways you can sync Drupal accounts.


Using Drupal OpenID-Single-Sign on (OMniauth) which very common method for the big organisation who manage a number of drupal/websites. more on that please refer to Comparison of Single Sign On (SSO) Modules
You can also look into Account Sync module which I haven't tested myself yet and it has a very limited number of site "130" reported using it so you better be careful with it. The module provide basic single sign-on support.
The other way will be to export users from sites using User Export module. then use Feeds module to import site. You can use Rules module to to export and import users between both sites, probably by running crone on both sites in sequence.
Take look into Bakery Single Sign-On System, which is useful single sin-on for the sites that based on the same second-level domain "example.come, test.example.com".
Domain Access module which suite of modules that provide tools for running group of affiliated sites from on drupal installation and single shared database. The module allows you to share users.