After this question http://www.question2answer.org/qa/24768/how-to-integrate-2-q2a-sites-with-one-wordpress-blog I thought I should create a video tutorial for this and just done here.. If anyone interested..
With this user will have to register only onece and they will be able to use all q2a website you have setup in network.
TV Q2A Market
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Jan 12, 2019: Q2A 1.8.3 released!