As to the question, if both are watching "live" then they each require a tuner, regardless if they are watching the same channel. However, if the Genie were to hit the R record button, then both could watch the recording without using a second tuner.
If it were me, I'd simply replace one of my HD DVR's with a Genie and keep the other HD DVR's. I might put a Genie Mini in my bedroom, but I'd keep the HD DVR's in the other locations.