Cross posted thread

[Admin]

You can create another column in the thread table, let's call it "pointer".
There, you could keep all of the forum IDs in which you want that thread, i.e "1 2 5 97", where each number is an ID of a certain forum.

Then you can use that value to select threads in other forums, you can add something like "OR pointer LIKE '%$forumid%'" to the WHERE clause of the query that selects the threads.

Just an idea.