I don't think that second nav menu is really necessary. But the only way I can think for that to work is to ditch the iframes and put all the content on separate pages. I'm sure there's some JS out there somewhere that can accomplish your goal though. Maybe Flash, too?
Oh, and just a suggestion after glancing at your code: take a look at XHTML because most of your coding won't work down the road.
http://www.w3.org/TR/xhtml1/