PhD selection discrepancy
Unless Prof B has made you a firm offer in writing, for a PhD, then he is being unreasonable. This is a terrible situation to put you in: he wants you to commit to him, withdraw all your other applications, whereas he has obviously not decided yet that he is going to take you as a student. If he takes you, then no harm done; but if he takes someone else, you're screwed. Nobody should apply to just one job... or even just two! The risk is too great. Everybody should understand this, and someone who doesn't understand isn't somebody you want to work with.

However as Dawn says in the comments, you should try to clarify the situation. Explain to Prof B that the interview with Prof A is just an interview, not a firm offer; exactly like the one with him. If he still insists that you only go to one interview... I would think very hard about continuing with him.