2
Why in the world would you use Apache to proxy to IIS?! That's like using your best cookware to make dinner then serving it in on paper plates. If you're going to be running .NET on your site, then I can see needing IIS, but even standard ASP will run on Apache.
But I digress...
Did you read Apache's
mod_proxy documentation?
As blasphamous as the idea is to me, seems you're not the only one who has done it, so you might find the following
Google Search helpful.
Sorry I don't have a direct answer for you, but what you are doing is a little unusual. Hope the pointers get you going in the right direction.
Insanity can be defined as "doing the same thing over and over and expecting different results."
Stupidity is not a crime. Therefore, you are free to go.