I tried the virtualbox installation of Nethserver with 2 NICs and it worked like a charm. With VMWare player I had to play(enable/disable/set ip/dhcp) with the host-only adapter in VMware and in the OS to make it work. But in virtualbox everything worked out of the box. Here is my virtualbox config:
Don't forget to allow promiscuous mode in the advanced network settings of the host-only adapter:
Nethserver then started with two green DHCP interfaces in different networks(my LAN and the host-only network), so I changed the WAN one to red, the LAN one to static and that's it.
But as you have two seperate networks bridging both adapters is ok, the config with one bridged and one host-only interface is just for test cases to have Nethserver as real firewall working behind a router in a LAN and not to be the router itself.