I've been running a site for a while now, with two separate WordPress installs for two separate purposes. An example would be one solely for political issues and the like, the second would be a more community driven purpose, such as book and movie reviews, and other recommendations.
I'm about to 're-invent' the purpose of the sites for various reasons with several of my fellow members.
What I'm wondering is if there is a method to use both WordPresses with a single user ... thing. Not sure what to call it. What I mean is if you register on one, it will carry over to the second WP as well.
Also; Is it possible to do the same for other software? Like vBulletin forums or Coppermine Image gallery?