The RTL can be paired to many devices the same time so you could have it connected to the RDU as well as an Edge 25/510/810/520/1000.
On the Edge you would get the radar traffic display as well as the audio tones so I'm not sure you would need the RDU as well. Can configure the Edge to give the radar display and not audio alerts, but there is not a way to configure it to give you the audio alerts without the radar display.
I ride a tandem and it can be difficult to check behind sometimes and therefore bought the radar unit to assist. I have mounted the RDU on the back for my wife (stoker) to keep an eye on, and use the Edge 1000 up front (Captain!) with audible alert. Works great.
