I have a client who is using multiple WordPress installations on the same domain, in subfolders to act as categories, but each with their own database. Each install is running independently. So plugins, and users are not shared. Each individual install has it's own users, and my client would like to allow users from different installs be able to login using the same details. I have read how to achieve this with fresh installs, but I am unsure how to make this change to an already active site.
Something that I wondered about as well, if anyone could answer, does this method of multiple installs, and multiple databases help with the performance of WordPress?