2 copies, 2 PC on a NAT

Hi
i want to be able to play MP at the same time on 2 PC behind the same NAT but I am aware that only one client can connect to the default port at any given time, so I was wondering if i could change something then I came accross the readme and it says "Default value for 'game' is 64100 (can be adjusted).".
So how do I adjust this?
Thanks