Shared users and sessions between blogs

I would like to share all users between 2 different blogs, maybe even keeping the sessions between them. Just as I am logged in to Gmail if I login to Google/Picasaweb/etc.

I got the tutorial of Justin Tadlock, but it is very old and not updated: http://justintadlock.com/archives/2008/09/20/installing-two-wordpress-blogs-with-the-same-users

Can it be done in WP 3.4.+?