Just be honest really. It may not be even necessary to mention that you've been in communication with Prof B before - just talk with Prof A about his or her research and how your interests fit, and if it becomes relevant during the conversation, then you can mention that you were in contact with Prof B before the deadline. Either way I don' think it would hurt you - it's not like you've made a commitment to Prof B before, or s/he made one with you.