According to Ruger's website: There is a New Model Single Six and a New Model Single Six Convertible. However, the website also states that "all .22 caliber models come with two cylinders (one in .22 LR and one in .22 Magnum) for increased versatility."
Ruger has made the ''new model single six'' with one cylinder or and with 2 through out the years. From what I understand if you want a second cylinder you need to buy one after market then send it to Ruger to fit it. from what I hear it is easier to buy a new gun.