You get one DVR -- it's a dual tuner device -- you can record two shows simultaneously just like the regular DVR.
With HomeDVR, they enable a feature in the device which allows any other STB in the house see any program which has been recorded by the DVR. You can play up to two streams (one HD and one SD) simultaneously -- this goes over the MoCA ethernet network which exists in your house to allow the STB's to get their Guide and VOD data.
Your other STB's basically are read-only devices able to see your main DVR.
I use it to allow me to watch shows that I've recorded from my office/bedroom while my kid might be monopolizing the TV in family room (where the DVR is located).
Please note the downside is you only have pause/rewind/fwd capability on a recorded show. You can't initiate recordings from the STB if its not a DVR on its own. If you chose to get a DVR for the extra room them they will not be able to communicate with each other.