multiple website, multiple database , different register form but same login

Hi all,

I have to create 2 websites that serve different purposes.

For the first website (eg abc.com), I am currently using the core module from Drupal 6 called the profile module to create the registration form.

Right now, I need to create 2 types of users, which is basic user and VIP user.
Each type of user will need to have:
a) a different registration form OR
b) different field to fill in (using the same registration form) after the select their user type

Upon registration, they will need to have access to different content within the same website.

My question is, how can I make the system differentiate the different users and direct them to the correct content?

For the second websites(xyz.com), I need to inherit the users from abc.com. On xyz.com, the different users will have different level of permissions which will enable them to perform different tasks within the website.

In this case, the basic user will be given the permission to take surveys, whereas the VIP user will be able to create surveys. However, the VIP users will not be given permission to publish the surveys that they have created until they are approved by the website administrator.

Can someone help me? How to do this? Should I use multi-site or is there any other module(s) that will let these 2 website's share the user database?

(p/s: the abc.com is a running site but xyz.com hasn't been create)

Thanks in advance!