DMZ - Question

[hol64 () hotmail com]

I have to setup a DMZ on our network. Our current layout is Internet Router <--> Firewall <--> WAN/LAN Router <--> 
Servers

The idea is to setup a back-to-back DMZ or Dual Firewall DMZ. So the topology would be like this..
Internet Router --> FW-1 <--> DMZ <--> FW-2 <--> WAN/LAN router. 

On the DMZ we will have a Web Server that needs access back to the Mainframe on the LAN, and a Mail server that need 
access to another mail server on the LAN.

One of my questions is the DMZ is in a /24 subnet and the LAN is on a /16 subnet. Is the only way for the web server in 
the DMZ to communicate with the inside LAN by NATting in the FW-2. Isn't this creating a double subnet from the 
outside??

I am working with 2 pix firewalls, and I am hoping to change FW-2 to a different brand that has stateful inspection.
 

Please Advice,

Thanks, 

Pablo