Push a UIImageView with another UIImageView, possible?
- (void) didMove:(UIPanGestureRecognizer*)recognizer {
    UIView* view = [recognizer view];

    // Remember our UIImageViews are class members called, a & b
    //
    UIView* otherView = view == a ? b : a;

    if (recognizer.state == UIGestureRecognizerStateBegin
        || recognizer.state == UIGestureRecognizerStateChanged) {

        // Moving left will have a negative x, moving right positive
        //
        CGPoint translation = [recognizer translationInView:view.superview];
        view.center = CGPointMake(view.center.x + translation.x, view.center.y);

        // Reset so we always track the translation from the current view position
        //
        [recognizer setTranslation:CGPointZero inView:view.superview];

        if (CGRectIntersectsRect(view.frame, otherView.frame)) {
            // Translate by the same number of points to keep views in sync
            //
            otherView.center = CGPointMake(otherView.center.x + translation.x, otherView.center.y);
        }
    }
}