I've been trying to troubleshoot this in a BuddyPress support thread, but I thought I might try here as well.
Basically, most of the functionality works, I already had separate networks, and the separate BP instances have their own groups and member lists. The issue is that if I update on one site, the URLs on the other sites point to that one. It is always the last one that is updated, all the other BP sites point to that site's URL.
Repeated from that support thread:
However, if I update something (for example, if I edit a page) on 3Cities, the profile URL then points to http://3citi.es/people/maiki/, and refreshing the directory on notes.vg shows that the link there now points there as well, http://3citi.es/people/maiki/.
Any ideas on why that is happening?