just I want to ask how set DotNetNuke portals to use login information of Site A to login to site B?
Out of the box, in DNN, if you want to do this, you need to use the Site Groups functionality in the PROFESSIONAL edition of DotNetNuke.
That being said, it is possible to make this work in the community edition of DotNetNuke, but basically you need to add a record to the User Portals table for every user.
Assuming all users are in Portal A, then you would want to add a record matching them up to also be in Portal B. You will also want to add the user to the proper roles (Registered Users) in Portal B as well.
Doing all this would likely be easiest with a database trigger.