Adding a third DVR to the whole home network

Is there any reason why I can't add a six channel DVR to my whole home network which consists of two 8642HDC two channel DVRs? The eventual goal being to return one of the 8642HDC to Cox when I have finished with what is recorded on it. I understand that I will have to pay for a service call to get the six channel installed but does it require another service call to remove the two channel later on? Seems to me that I can just unplug it and take it back.