Connecting two Multisite database tables

Goal: Connect two separate WordPress Multisite installations together through table sharing, much like CUSTOM_TABLE_USER method of sharing user information.

Details: One Multisite will be setup up as a sub-directory install, the other will be setup as a sub-domain install with domain mapping to unique domains.

Sharing user information is critical for our network of sites to function well. Unfortunately (or fortunately) we have a few unique domains that need to share both users, and potentially, need to be setup to share posts. We’re going to use the plugin ThreeWP Broadcast to share blogs that smaller, more user-centric sites to share with our larger sites. We’re trying to avoid duplicate content SEO problems by forcing shared content back to their original permalinks. The only way I know how to do this well is through ThreeWP Broadcast.

So, what we’re hoping to do is create something like this:

http://site1.com/sub-directory1 would be connected to the table at http://sub-domain1.site2.com/.

http://site1.com/sub-directory2 could also be connected to the table at http://sub-domain2.site2.com/ and so on.

One of the few directions I found online was this blog entry: http://raventools.com/blog/how-to-easily-manage-multiple-wordpress-sites-with-one-database-and-one-code-base/

Any help or words of caution would be most appreciated. Thanks in advance.