I've successfully installed 2 separate wp install which share their users and user_meta tables. I've done this by making changes which can be found here:
An extra note to these changes: Go to both (depending on the number of wp installs running, in my case 2) option tables and find the option with option_name [your_prefix]_user_roles and rename this to user_roles. If you don't do this no user will be able to log in.
Now my question. Is it possible to share the session so that an user doesn't have to login again when switching from one install to the other?