Default Gateway Issue

I have a network setup as following:

WAN1 <----> Router -----------------+
(192.168.1.1) |
+------------ Server 1
| (IP: 192.168.1.10)
| (GW: 192.168.1.1)
|
+------------ Server 2
| (IP: 192.168.1.20)
| (GW: 192.168.1.2)
(DMZ) +
WAN2 <-------> Untangle -------------
(192.168.1.2)

Case 1:
With the above setup,
I can access Server 1 from WAN 1.
I cannot access Server 2 from WAN 1.
I can access Server 2 from WAN 2.
I cannot access Server 1 from WAN2.

Case 2:
Server 1 GW set to 192.168.1.1
Server 2 GW set to 192.168.1.1
I can access Server 1 from WAN 1.
I can access Server 2 from WAN 1.
I cannot access Server 2 from WAN 2.
I cannot access Server 1 from WAN2.

Case 3:
Server 1 GW set to 192.168.1.2
Server 2 GW set to 192.168.1.2
I cannot access Server 1 from WAN 1.
I cannot access Server 2 from WAN 1.
I can access Server 2 from WAN 2.
I can access Server 1 from WAN2.

Case 4:
Server 1 GW set to 192.168.1.2
Server 2 GW set to 192.168.1.1
I cannot access Server 1 from WAN 1.
I can access Server 2 from WAN 1.
I cannot access Server 2 from WAN 2.
I can access Server 1 from WAN2.

Case 5:
With a PC connected as 192.168.1.30, both servers can be access
no matter the GW was set to 192.168.1.1 / 192.168.1.2

Any suggestion on why it is so?????
Tks.