I have never heard of a consumer or SOHO router that can port forward a port to more than one IP address. There may be some Pro level routers that could do this but it would take some programming. What people usually do to get around this limitation is to either have more than 1 WAN side IP address so that ports can be forwarded on each incoming IP address separate, or (depending on the game) you may be able to change the port the game uses to another port on one of the PC's. Then you would have 2 different port numbers you could forward to 2 different PC's.
It looks like you may be able to change ports on one PC. Take a look here:http://support.activision.com/articles/en_US/FAQ/Lag-Occurs-in-Multiplayer-Game/?l=en_US&c=Game_Title%3ACa
By the way the article above talks about turning off all your routers firewall features. I would not do that! Try the other things first to see if it solves your problem.