BuddyPress.org

I have a few questions about taking my sites to the next level by enabling multisite and networking them.
Right now I have a large buddypress community and I have almost completed my second for opening. I am wondering if I should create a new site that is the portal to these two sites and acts as the master site for the network. Is that a good approach? or should each of these sites be their own master of the network?

Does this approach make my members already registered for the new site? Do they have to login only once? Can they change their name or profile details on one site to be different than the other?
When I enable blogs on multisite, does everyones blog updates post on the site and then when you click on them it takes them to their own wordpress themed blog?

Is there any video’s or examples people know about to visualize this better?

Any help would be greatly appreciated! just trying to wrap my head around it all as I think it is the next huge leap for my sites.