Anyone Aware of a Plugin for Sharing Users Between 2 Standard WP Installs on Same Domain?

Hi all,

I have two separate standard WP installs, one at http://domain.com and one at http://sub.domain.com. Multisite is not an option in this setup.

I would like to share one user table between both installs. Is anyone aware of a plugin to do this? All I can find in researching so far involves editing the wp-config and core or a plugin that hasn't been updated in awhile. http://wordpress.org/extend/plugins/shared-users-reloaded/

I'll keep looking but thought I'd ask here in case anyone else is doing anything similar with a plugin I'm not aware of..