For Premium Channels, the card needs to be paired with the Sky box that it is being used with and only that box.
If you want to use more than one box you need a Multi-room subscription. (It used to be about ten quid extra.) This will allow you to have more than one box with Premium channels in the same dwelling by supplying extra Viewing Cards. It will not allow you to have them in more than one dwelling however.
T, September 2010