Just like LK's code says. Your exact code depends on your shared forum numbers.
At the end of both ubb_new_topic.cgi and ubb_new_reply.cgi you'll find:
[/code]This needs to be changed to:
[code]
Replace 'SPECIAL FORUM #' with the number of your shared forum. If you have more than one shared forum, you'll obviously have to change the code to handle more than one number, like with an 'OR' or whatever works for the number of forums you are sharing.