its fairly straightforward to set up master slave replication, assuming you've got that level of access to the server, which would make sure everything added here gets added to the other server.
difficult thing would be how to deal with the other direction. you can use replication in both directions, but the servers need to use offset auto increments (one uses odd numbers, the other uses even) and things, to ensure that they don't both add data with the same primary key. this would mess up the thread/post numbering though, since it relies on primary keys.
only way i can see it working is if use 2 way replication, but use a script to prevent people accessing the backup forum unless this place is down.
but such a script would need to be fairly robust, or it may not let us in when we need the backup place, and equally, may let people in when it shouldn't.
so, probably more effort than its worth!