Same login for 2 wordpress installations

Hi all,

Forgive me if the post is a dublicate one as in the previous posts I could not get an answer.

So I have a website which runs on wp. Now I want to set up a video gallery type of thing which will be on videos.mysite.com.
Here I want the users to login only once either to the videos site or the main site.
If logged in through the main site and surfing through the videos, they should be able to like or comment using their same logged account only.
I did tried few video gallery plugins but they did not meet my requirements for the video site. So thought of going it this way.
Appreciate help here.