cards are registered to the owner not the box. you can record and use a card with any sky plus box as long as the card's pin is verified with sky. the box only checks the server if the card's pin is allowed to view channels and have the record function.
when i used to repair sky boxes for a living we could use any card to test any box, we had a subscription card and used to share it around to check the boxes were working.
it might have changed since HD has been out for a while, but I remember repairing HD boxes and this was still the case.
You could get away with using a twin lnb rather than a quad, as this is just repeating the number of signals you can receive. if your not going to use sky in different rooms you could just use a twin tuner. you need another cable running through like scott said so you can record one channel, and view another. technology currently cant run 2 different channel signals down 1 cable.