How can two machines play Demigod if you are behind a router

I have a LAN in my house, and a router. usually, i just set it to DMZ mode, exposing one of my computers to the internet (so that all packets going to my external IP get to my main PC). 

Now if i understand correctly, to play over internet i need connections in both ways, so people from internet will have to connect to my other computer, if i wanna play with a friend.

So how do i set port forwarding in this case? is it possible for two persons to play at once if they are in the LAN behind a single router?