In the body tag of your main page (bottom frame):
<body onScroll="topFrame.document.body.scrollLeft = document.body.scrollLeft;">
Here's an example where the frameset actually sets up the "scroll syncing". Useful for an application with many pages that you might need to have this capability:
http://www.geekdork.com/samples/dhtml/HSyncedFrames.htm
(Look at the frameset source for the exact code.)
Peter
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Work smarter, not harder.