The masters currently cannot communicate with each other, and the web interface can only treat them as separate master controllers. There are some potential solutions you may consider. If it’s possible for you to extend the wires going to the valves, you can use the OpenSprinkler zone expansion board, so that one master controller and several zone expansion boards will sit together, and the wires going to valves all start here. The other possible solution is to have multiple controllers listen to a server (for example, use a Raspberry Pi), and the server will send out http commands to every controller in order to have a centralized control.