Have you tried setting UIGestureRecognizerDelegate and handling the both gesture recognizes in similar fashion as described in FAO?
e.g.:
#pragma mark - UIGestureRecognizerDelegate
- (BOOL)gestureRecognizer:(UIGestureRecognizer *)gestureRecognizer shouldRecognizeSimultaneouslyWithGestureRecognizer:(UIGestureRecognizer *)otherGestureRecognizer {
if ([otherGestureRecognizer.view isKindOfClass:[UISlider class]]) {
return YES;
} else {
return NO;
}
}