The Observer system cannot use any other monitor to view the video from the cameras.  The included monitor with your system has all of the technology to communicate with the cameras and is the only way to display the Live view from your cameras.  The monitor does not have any video output port to plug it into another monitor.
An option would be to use a larger Android tablet or iPad to view the video from your cameras.  Just download the Observer app to the device and connect to the Observer system over Wi-Fi.