Howto configure 450G to have two LAN and one WAN in the middle

Router Config.jpg
Is it possible to configure the RouterBOARD 450G to have a common WAN area of three Ethernet interfaces which work as a switch (e.g. ether2, 3 and 4) and two independent LAN interfaces (e.g. ether1 and ether5) having the same local IP address and use NAT to connect to the WAN area?
Just like using two seperate routers and a switch that represents the internet, which btw works with two RB450G out of the box.