While obviously I haven't looked into this fully, nor do I have say over what features are added - I think it may be a little more complicated than you are thinking, this is just off the top of my head:
1) Searching - the search engine would probably need a bit of a rewrite to accommodate this
2) Permissions - what if they have permission for the alternate forum, but not the main one? Can they see the topic in the alternate forum? If no, it would be too resource intensive to work, if yes, well then, we have a bit of a permissions problem...
3) Moving - what if the thread is moved? Does it move from the forum it is moved from? Or both? What happens if you try to move the topic in the alternate forum to the main one? The move function would have to be rewritten a bit however that is decided.
4) Deleting - if you delete one, does the other delete? The delete function would have to be rewritten a bit.
5) What if someone wants to add an alternate forum after posting? We'd need something to accommodate that.
6) What if someone wants to remove the alternate forum after posting? Even worse, what if someone wants to remove the main forum and make the alternate forum the main forum? We'd need something to accommodate that.
Personally, I think it would be a bit of hassle for a bit of strange feature - in what circumstance would you need this? There might be a better solution, such as allowing announcements to have replies, maybe?