It could be numerous things. You could have transponder sensors out, which are placed in various locations in the vehicle that communicate with the smartcards, or you smartcard battery contacts might not be fully touching the batteries. Try taking them apart to find the problem. Try to get both to work just temporarily at least so you can buy more cards and program them yourself. To program a card yourself, you do have to have 2 working cards. If you can only get one to work, dealer will have to program a second, and then you can program a third. Dealer will charge about an hour of labor to program a second.
If you cannot get either card to work, and using aux keys alone is not satisfactory for your needs, then you have to but a new computer + 2 cardkeys and this will run you in the 800 dollar range.
I'm sure if you are dilligent, you can get both cards to communicate just for a minute or 2 while you program a third yourself. If you can get both of them to work, buy 2 more and program them yourself.