Dual iframe loading, or after load - refresh another into separate iframe target

Here's the basic layout of my site:
Horizontal navigation bar
sub-navigation bar
iframe content box


My presumed navigation would have the user click an image in the top Horizontal navbar, which would then load a list of subdirectories and choices in the sub-navigation bar.

I can do that - I'm there so far.
What I've been rummaging around the internet for the past few hours in search of, is -any- kind of code to, after the sub-navigation bar has loaded, to ALSO load a -separate- page into the -separate- iframe content box.

(To further break it down if needed, this is what happens with normal iframes)

Loading the page:
Navbar
further information/sub-navigation
content box


(click)Navbar
(littlepage loaded) further information
content box stays the same

What I want, is after the first page is loaded into the subnavigation, to also load something in the content box, so I have a sub-nav bar, and more info on that sub-nav bar.

Thanks ahead of time, and my apologies if I was not concise enough. I'll do my best to respond to any questions aimed at me.

funny how u should ask, i've posted a solution to something similar to your question http://www.codingforums.com/showthread.php?t=53764 and just to sum it up for ya, this is how a link would look
<a href="http://www.codingforums.com" target="top" onclick="parent.document.getElementById('main').src='http://www.yahoo.com';">
give the iframe an id and where u see getElementById('main') replace main with the id of the iframe, and just set the target of the link to the navigation frame