I had the same problem. I have two NICs. One for our home LAN, where the router is located and one for LAN-parties, or when a friend wants to get some stuff from my pc. You have to create a network bridge to get them both working at the same time. To do this go to network connections, select your NICs and choose create network bridge from the context-menu. Now you have a network bridge, which can be configured like a normal network card and is now used instead of the both NICs. Hope that will solve your problem.
