One of the things to do when transferring like that is to close out the first account like someone above suggested.
Another option would be to call the present card company and tell them what it is you are going to do and see if they are willing to match or at least lower your interest on the card you have now. They want to keep your business. They also are sending out such promotions to people to get new ones and very well may do it to keep old ones. It may not be O% but could be much lower and cheaper in the long run with lower interest when the promotion is done. I have called several times to have my interest lowered.