i would highly recommend that you ask invisionboard support or forums since how this is going to work is highly dependant on how the forum software was programmed. ask them if you can run the software on 2 different domains or file systems with the same database. if the answer is yes, then you will know if you should even proceed.
now that i look at your config file, a symbolic link would not work because it is has values that are sensitive to the domain it is running on.
anyhow, a symbolic link is just a 'shortcut' on a *nix system. it would sort of be the same as having an identical copy of the original file system in the second sub domain. you would need shell access to setup a symbolic link.
Code:
$ ln -s /path/to/source /path/to/shortcut
sorry i can't be more help.