- (void)viewDidLoad
{
    UITapGestureRecognizer *doubleFingerTap = [[UITapGestureRecognizer alloc]
                                                initWithTarget:self action:@selector(handleDoubleTapGesture:)];
    doubleFingerTap.numberOfTapsRequired = 2;
    doubleFingerTap.delegate = self;
    [self.webView addGestureRecognizer:doubleFingerTap];
    [doubleFingerTap release];
}
и
#pragma mark Gesture recognizer delegate
- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer 
{
    return YES;
}
так-же и сам метод, который будет вызываться при двойном тапе:
- (void) handleDoubleTapGesture:(UITapGestureRecognizer *) gestureRecognizer
{
    if (gestureRecognizer.state == UIGestureRecognizerStateEnded)
 {
...
 }
}
Ну и на последок, в *.h-файл нашего класса, добавляем описание поддержки протокола
<UIGestureRecognizerDelegate>
