How can I use BP_MULTIBLOG to accomplish my goal?

Hey. So I want to have the Activity feed on more than one site. BP_MULTIBLOG appears to do this, but i'm cautious. I'm wanting to understand how this works.

First off, I don't need buddypress on all the sites. Just on the main site for social and an Academy site, and perhaps another one or two sites.

The objective is simply to have users be on a particular site and have access to the activity feed on * that * site, segregating them into specific zones.

Example: Basic social networking on the main site; Academics on the Academy site, etc.

How can I set up MULTIBLOG on *specific* sites ONLY within the multiste?