SKY are only going to let a subscriber take out a mirror subscription at the address they have listed for their primary subscription. The T&C's then oblige you to connect both digiboxes to the phone line tied to that household to confirm that both smartcards are used at the same registered address.
Presumably if a wireless telephone extention were used to connect mirror to the main subscribers phone (or wired either way, depends on how close a neighbour) & sky dish etc were ready to roll at the neighbours using the mirrors box & card. Lastly Sky would presumably install the a mirror box at subscribers address. I don't see why you couldn't use the mirror box & card at the neighbours still connected to your phone. I'm sure it would work & Sky package for 1/2 price + £5 makes me wish I was your neighbour. I'm sure Sky & Bt would have something to say although I doubt they'd find out. It would have to be box & card as the card is tied to the box, & if you blagged Sky you'd changed the box it may raise suspicion.
Yes that is the weakness in the current security method. Fairly impractical for single dwellings but I imagine very easy to arrange for friends in adjacent flats or semis.
Of course you would have to trust the friend just incase they started making international calls or ordering PPV