Declare a custom validator method. Set the rule for one field to call the validator method and declare the second field safe.
Something like this:
In your model:
public function rules() {
return array(
array('field1', 'myValidator'),
array('field2', 'safe'));
}
public function myValidator($attribute,$params) {
if(strlen($this->field2)==0 && strlen($this->field1)==0 )
$this->addError('field1', 'You must enter either field1 or field2');
}