How to login a q2a user from using another domain? (Single Sign-On / Multiple Logins at once)

Question

Situation: 

• Running a q2a forum siteQ.com
• Running a custom website siteB.com with a custom script for user logins. 
• I want to get rid of the custom login script. Instead I would like to use the table "qa_users" from siteQ.com (q2a forum) for the login handling.
• I know already that I can access the q2a db and files from siteB.com via external integration. 

Wanted: 

1. User does his login on siteB.com using table qa_users (works already with external integration)

2. User coming from siteB.com visits siteQ.com which recognizes the user as loggedin.

By the way, you know this login feature from Google and Stackexchange, for instance.

Question: 

How can this be implemented?

.
 

I researched already and found on stackoverflow: 1, 2, 3. But probably there is a simpler way within question2answer.

Comments

I thought I could just do from siteB.com: <form method="post" action="https://siteQ.com/login"> BUT the q2a forum expects the security token.

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Are the 2 domains on the same server?

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Yes, full root access and same database.

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I think the way to achieve this is by using AJAX. I found an example here and will try to test it soon: https://github.com/0k/multidomain-sso