My first answer may need clarification. In fact there were two questions
in your mail:
#1 - How do I make the WAN interface responds to ARP queries for
additionnal IP address?
#2 - How do I set up NAT for the DMZ services?
>>Proxy ARP is your friend.
I admit I've focused on #1 and simply forgot #2 :-(
I dont know how you get this 2 IP addresses, but in most cases you don't
need Proxy ARP and I think 1:1 NAT is what you need.
>>Read the Doc!
A more useful answer would have been: