css_iframe.html

<div style="width:100px; height:100px; overflow:auto; border-style:solid">
  aaa<br><br><br><br><br><br><br><br><br><br>
</div>

css_iframe_two.html

<div id="hidari" style="width:100px; height:100px; overflow:hidden; float:left; border-style:solid">
  aaa<br><br><br><br><br><br><br><br><br><br>
</div>
<div id="migi" style="width:100px; height:100px; overflow:auto; float:left; border-style:solid">
  bbb<br><br><br><br><br><br><br><br><br><br>
</div>
<div style="clear:both"></div>

scroll_both.js

$(function(){
  var migi = $("#migi");
  var hidari = $("#hidari");

  migi.scroll(function(){
      hidari.scrollTop(migi.scrollTop())
  });
});