OK, not the usual "how do I renew...." type question.
PCWorld (uk) is currently offering Norton Security Deluxe 10 devices at a silly low price, so I've bought two cards. Obviously when I put the number in from one of them I'll get 12 months, but what happens if I then immediately put the other number in as well? 24 months or would I "lose" one of them?
If I can just keep adding to build up a few years subscription it'll save me a fortune as I'll just go get 4 more and then I can forget it for 6 years.... :)