Local redirect for 404?

I have 2 web servers sat behind a dlink604t router (we'll call them server a and server b). Both servers are running win 2000 advanced server with IIS. Port forwarding is set up on the router to forward all web traffic (80) to server a.
What I need to know is this... If server a returns a 404 error can I get server a to check server b for the requested page and return it if found? How do I do this?

1. Why are you running a web server behind a soho router?

2. Why dont you just put the page on server A?