Why should hearing aids have two microphones?

Most reasonably priced hearing aids use a single-microphone design -- because one microphone is cheaper than two. But dual microphones can mean the difference between "hearing" and "understanding." A two-mic hearing aid can literally turn down sounds coming from in back of you -- so in a noisy restaurant, you hear the people at your table, not everyone in the room. Here's a very good article explaining the history of dual-microphone hearing aids, going back to the pre-digital era. It's somewhat technical, but loaded with information. But the bottom line is: two microphones are better than one. (Which is why ZVOX does not offer single-microphone hearing aids.)