I want to create two different type of users for example internal and external users. For internal users need to be authenticate against different database. Any suggestions?
Unless you wish to write a custom module with direct access to the second database, with its own login system, the only way this is going to work is with direct manipulation of the drupal database, renaming tables, and using 'Create View ...' to give you access to the information. The amount of work, maintenance problems, and security implications would be horrendous.
If the number of 'internal' users is fairly static, my suggestion would be to use the User Import module, and give them all a role of 'Internal', using the user permissions to control their abilities on the site. Any new Internal users could be added manually.