This a stupid question but Im just checking. If a place sells double backed cards red/blue is that 26 red and 26 blue or red on one side and blue on the other?
Usually red/blue will indicate that a full deck would be 52 cards, red one side and blue the other. I was recently told that on some sites there might only be 40 cards in the 'deck' so it might be worth checking before placing an order online.
Check out cards4magic.co.uk as they can usually offer a variety of 'special' cards and decks - you can always phone or e-mail Clive and he might be able to do something particularly suited to your requirements.