I'm still on Sky+ HD, so not Q yet, but I think the principal is the same. I have a 2nd sky box in the loft and use a HDMI modulator to put a HD onto the freeview scale and sent around the house.
so I believe you could put a mini in the loft in the same manner. And in theory its easier as you don't have to run the satellite cable into the loft like I have as the mini attaches to the Q wirelessly. So would work as long as you wifi signal between the 2 boxes is good enough