The only simple way of doing this is to set the second web server on a non-standard port, and set port forwarding up on that. This is probably unsatisfactory, so I would suggest you review the reasons why you need two separate boxes to run as webservers (eg., can you 'consolidate' this to one, and use virtual hosting?).
Otherwise, you may have to look at changing your firewall to some sort of (transparent?) proxy.