jQuery validation, two field fill one.

http://bassistance.de/jquery-plugins/jquery-plugin-validation/
Hi, if I have two field, I want if both field was empty, show warning, if one of them has been filled, then pass
how to do that?

<input name="phoneDay" type="text" id="phoneDay" />
<input name="phoneNight" type="text" id="phoneNight" />