@REGE: yes it does seem like you need two zone expanders, one for each main controller. If both controllers are able to connect to the WiFi network, then you can make use of the remote station feature like Mike mentioned to manage them together. The basic idea is that the first controller will be the master controller, and the second will be a remote controller. On the master controller, you can set 36 zones (even though it only has 24 physical zones), where the last 12 are configured as remote zones that map to the second controller. On the master controller, if you switch on/off any remote zone it will trigger the action on the remote controller for the corresponding zone. Hope this makes sense.