It has to be a paper travel card - bought at a national rail station - these can be attached to the tube, but arent available (with a few exceptions) at tube ticket halls. Tubes sell Oysters, which cannot be used for the 2for1.

just a small comment. if you get the 7 days travelcard (paper or not) you can use both peak and off peak hours without any additional cost. and even if you get one for zones 1-2, it includes buses for all zones