You can't do this with CGI, or any other server-side mechanism. HTTP works by call and response: the browser requests content from the server to fill a particular slot, and the server provides. The solution, therefore, is to make the browser ask for the other frame as well.

Which is a (boo) javascript thing. When you return the main frame, put something like this in the header:

<script language='javascript'>
top.lefthandframe.location.href = '[url]';
</script>
[download]

This will of course only work for people whose browsers can do (simple) javascript, and who haven't switched it off, but in most cases if the browser can do frames it can do this.

The only alternative is to make your script write a new frameset first. That's more reliable, but if you end up with three cgi invocations to make one page, then it's time to reconsider the page design.

and anyway, if you're changing both frames at the same time, why not just put them on one page?