Share discussion post accross community sites

I am facing this requirement with the community sites that users from 2 different community sites want to access the post posted in one's community site.

Let me simplify this. consider there is CommunitySite1 & CommunitySite2, I am owner in CommunitySite1 and I have posted a post(in Discussion List) in my CommunitySite1 now I want to give permission to a User in CommunitySite2 to view & reply my post in CommunitySite1.

Is there anyway to implement this and Share my post with member in other CommunitySite.