MultiDB: shared users table, default global tables

I have a slightly unusual setup, where my main site is on one wordpress (multi-site 3.0.1) install, and then I have a second wordpress install that hosts a buddypress install and user blogs. The two wordpress installations currently use the same database and share their users tables so that the users can use the same account in both places.[1]

However, I've got a large number of users and user blogs and I need to move the buddypress/users install to a multi-db setup, but it still needs to share the users table with the other install.

Has anyone happened to set something like this up before?

Can I make the global database the original database that both installs are currently sharing? Or another way to ask that I guess would be if I can use a totally custom name for the global database?

Ok, Thanks for any input.
B

[1] I honestly can't give very good reasons for this set up. The decision was made by a different developer.