How much information should I reveal in a postdoc interview, when I am also applying for a fellowship at a different institution?
They will probably ask you if you are applying or evaluating other positions during the interview, if they don't you don't have to bring it up.

But if they do ask, just say you currently don't have any offer in hand but that you are applying to other positions. When and if they make you an offer, at that point you should decide to accept it or not, or to ask for extra time to make your decision. But switching post-doc half way will not be good for your CV or your research, but that's another question.