Not hard to do. These were the steps I took ...
1. Installed minibb forum
2. Uploaded minibb forum software to new forum directories WITHOUT installing
3. Edited the _install_mysql.sql file found in the root of each new forum removing everything bar the tables for forums, posts and topics
4. Altered the remaining forum, posts and topics tables in _install_mysql.sql to reflect the name of the new forum, such as:
CREATE TABLE minibbmusic_forums
CREATE TABLE minibbmusic_posts
CREATE TABLE minibbmusic_topics
5. Installed the new forums
6. Edited each forum setup_options.php file to reflect the new table names such as:
7. Altered the cookie path to the ROOT of the domain such as:
That was all it took. A breakdown of the above is quite simple. Steps 1-6 allocate new tables for forum, posts and topics, whilst ALL other database tables are common. Step 7 forces ALL the forums to share the same cookie, which holds the logged in or out information. Handily, registration on one forum registers the user for all.