If you check out the link Bill posted from the start it is about connecting the Boredom Projects board via USB, that method for connecting multiple devices to one Pi running emonHub is discussed further in the Does the emonTx Shield need an actual Arduino? thread, since a Due is already USB I suspect this would be somewhat easier than using emonTx’s. An advantage of USB “serial-direct” devices is that you can upload firmware, reset the devices and power them directly from a RPi.
A couple of Boredom Projects boards (or more) connected via USB to a PI could give you a pretty tidy and powerful monitoring solution with lots of channels.