Manual:$wgCookiePath

Details
If you do not want your login to be persistent across Wiki's running on the same server, and your installations all use the same database, you will need to modify the LocalSettings.php file $wgCookiePath = " /directory "; Where directory is the directory for that MediaWiki installation relative to the DOCUMENT_ROOT.

This resolves issues where a user is occasionally prompted to log in when viewing a different installation, even though they logged in earlier and asked to have their password remembered.

session.name provides an alternate solution.

This workaround is not necessary for version 1.6 and greater.