BuddyPress.org

This is truly a question for the WPMU forums.

But if you look at the wp-config.php file, there is a constant defined for the overall site blog:

define('BLOGID_CURRENT_SITE', '1');

You can change that to:

define('BLOGID_CURRENT_SITE', '2');

But, first make a copy of your original wp-config.php file just incase something goes wrong.