I think (but am not sure) that the two CPU racks is in some kind of redundant configuration via the IM304-IM324R.
I would believe that you could just use the regular TTY cable connection to the PG port on either CPU.
Comments:
1. Do not try to make any modifications to this system unless you know exactly what you are doing !
2. As soon as possible, this system should be totally replaced by something newer that has a chance of being supported. I think a candidate is S7-400H in a redundant configuration.