[_ck_]

As long as they all point to the same user table and usermeta table, you can share the login across as many domains as you’d like, the cookies simply won’t be sync’d (however I have a plugin coming out soon that will share cookies across domains, but it’s not meant for wpmu, just regular wp/bbpress).

Database secret is covered near the start of this topic, it’s hidden in your bb/wp admin menus.