^^^ Telboy's problem, Tuvok, is that he's got TWO Sky+ boxes (but only one Sky+ account) and he's trying to get them BOTH to recognise the cards in them as valid.
Personally I've got my doubts as to whether it's actually possible because, if it was, two people (not necessarily in the same household) would be able to share the same Sky+ account.
There are references on the Sky website to 'Sky Multiscreen' (which seemed to offer what Telboy is seeking) but clicking through on the links now only goes to information about Sky Q boxes.