Have you looked into the changing the cookie prefix for the sites? (Server | Session Handling) - I'm a 100% Moodle newbie, just happen to see something on this in a book. Hope this helps...
Moodle networking (MNet)
Multiple MNet networking on the same server
ការពិភាក្សាត្រូវបានបិទ ដូច្នេះអ្នកមិនអាចឆ្លើយតបទៀតទេ។