PanelDue isn't designed to support so many heaters. However, my understanding is that you are actually only using 2 heaters, which are the heater 6 and 7 outputs. So one solution is for you to switch to using RepRapFirmware 3, which allows you to map any heater to any available port. This means you can map heaters 0 and 1 to outputs 6 and 7. See https://duet3d.dozuki.com/Wiki/RepRapFirmware_3_overview if you want to try this.