Hello friends,
I want to develop a feature just like "google gmail and google drive"
i have to domain one main "www.example.com"
and other is “drive.example.com” and both having same user/password.
so if i logged in to “www.example.com” and here is a button “Go To Drive” then on click the button i want to open a new tab for “drive.example.com” that direct access skip login.
I got the solution.
here as well: please give out a bit more details about what you did and where… so that other could participate in your solution 
Next Cloud login page accept GET and POST method by GET method its show the login form page
and by POST processes to login.
So i have making change on " “www.example.com”, here a button “NC Drive” on click sending postdata (user,password) to http://drive.example.com/index.php/login.