Making two UIScrollViews follow each others scrolling

Set the delegate of scroll view A to be your view controller... then have...

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
  CGPoint offset = scrollViewB.contentOffset;
  offset.y = scrollViewA.contentOffset.y;
  [scrollViewB setContentOffset:offset];
}

If you want both to follow each other, then set delegate for both of them and use...

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
  if([scrollView isEqual:scrollViewA]) {
    CGPoint offset = scrollViewB.contentOffset;
    offset.y = scrollViewA.contentOffset.y;
    [scrollViewB setContentOffset:offset];
  } else {
    CGPoint offset = scrollViewA.contentOffset;
    offset.y = scrollViewB.contentOffset.y;
    [scrollViewA setContentOffset:offset];
  }
}

The above can be refactored to have a method which takes in two scrollviews and matches one to the other.

- (void)matchScrollView:(UIScrollView *)first toScrollView:(UIScrollView *)second {
  CGPoint offset = first.contentOffset;
  offset.y = second.contentOffset.y;
  [first setContentOffset:offset];
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
  if([scrollView isEqual:scrollViewA]) {
    [self matchScrollView:scrollViewB toScrollView:scrollViewA];  
  } else {
    [self matchScrollView:scrollViewA toScrollView:scrollViewB];  
  }
}

Swift 3 Version:

    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        if scrollView == scrollViewA {
            self.synchronizeScrollView(scrollViewB, toScrollView: scrollViewA)
        }
        else if scrollView == scrollViewB {
            self.synchronizeScrollView(scrollViewA, toScrollView: scrollViewB)
        }
    }

    func synchronizeScrollView(_ scrollViewToScroll: UIScrollView, toScrollView scrolledView: UIScrollView) {
        var offset = scrollViewToScroll.contentOffset
        offset.y = scrolledView.contentOffset.y

        scrollViewToScroll.setContentOffset(offset, animated: false)
    }

I tried the Simon Lee's answer on iOS 11. It worked but not very well. The two scroll views was synchronized, but using his method, the scroll views would lost the inertia effect(when it continue to scroll after you release your finger) and the bouncing effect. I think it was due to the fact that setting the contentOffset through setContentOffset(offset, animated: false) method causes cyclic calls of the scrollViewDidScroll(_ scrollView: UIScrollView) delegate's method(see this question)

Here is the solution that worked for me on iOS 11:

 // implement UIScrollViewDelegate method
    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        if scrollView == self.scrollViewA {
            self.syncScrollView(self.scrollViewB, toScrollView: self.scrollViewA)
        }
        else if scrollView == self.scrollViewB {
            self.syncScrollView(self.scrollViewA, toScrollView: scrollViewB)
        }
    }

    func syncScrollView(_ scrollViewToScroll: UIScrollView, toScrollView scrolledView: UIScrollView) {
        var scrollBounds = scrollViewToScroll.bounds
        scrollBounds.origin.y = scrolledView.contentOffset.y
        scrollViewToScroll.bounds = scrollBounds
    }

So instead of setting contentOffset we are using bounds property to sync the other scrollView with the one that was scrolled by the user. This way the delegate method scrollViewDidScroll(_ scrollView: UIScrollView) is not called cyclically and the scrolling happens very smooth and with inertia and bouncing effects as with a single scroll view.

Swift 5.4 // Xcode 13.1

What has worked flawlessly for me was the following:

1. Create a custom subclass of UIScrollView
2. Conform to UIGestureRecognizer delegate
3. Override the gestureRecognizer(_:shouldRecognizeSimultaneouslyWith:) GestureRecognizerDelegate method

public func gestureRecognizer(_ gestureRecognizer: UIGestureRecognizer, shouldRecognizeSimultaneouslyWith otherGestureRecognizer: UIGestureRecognizer) -> Bool {
        return true
    }

4. Both collection views need to have the same superview. Add both gesture recognizers to your superview

yourSuperview.addGestureRecognizer(scrollView1.panGestureRecognizer)
yourSuperview.addGestureRecognizer(scrollView2.panGestureRecognizer)

Hope this helps!