Plain ol' Javascript that places a horizontal scroll bar at the top of a horizontally-scrollable element.
Add DoubleScroll.js via a script tag. Then, in your HTML, wrap the element you want to horizontally scroll with a div tag:
<div id="doublescroll">
.
.
.
</div>
Then in your javascript:
DoubleScroll(document.getElementById('doublescroll'));
You do not have to id your outer div 'doublescroll'. Name it whatever you want. Just reference it appropriately in the javascript call.
It's not required to style the #doublescroll element, but it might be advisable to do so in certain circumstances. For example, adding a border:
#doublescroll {
border: 1px solid #999;
}
That's it.