As long as both of your viewers are the same size, the images that they contain are the same size, and both controls are at the same zoom level, the code below will work:

<cc1:WebImageViewer ID="WebImageViewer1" runat="server" Height="400px" Width="300px" />
<cc1:WebImageViewer ID="WebImageViewer2" runat="server" Height="400px" Width="300px" />
   
<script language="javascript" type="text/javascript">
   atalaInitClientScript('OnPageLoad();');
   function OnPageLoad(){
      WebImageViewer1.ScrollPositionChanged = Viewer1Changed;
      WebImageViewer2.ScrollPositionChanged = Viewer2Changed;
   }
  
   function Viewer1Changed(){
      WebImageViewer2.setScrollPosition(WebImageViewer1.getScrollPosition());
   }

   function Viewer2Changed(){
      WebImageViewer1.setScrollPosition(WebImageViewer2.getScrollPosition());
   }
</script>