BuddyPress.org

Here is my thoughts:

1) It is already easy enough to share wp_users across several WPMU installs or WP installs, so that part would be pretty simple (even using HyperDB)

2) Having two WPMU installs being able to write to the same BP tables might be a disaster, but it might work. Once again using something like HyperDB, but it seems as though BP has issues with tables not being in same database as main WPMU tables.

I am still playing with this myself on test installs, so if I learn anything new, I would be happy to report.

Trent