You can't forward the same port to two different machines. You CAN forward different ports to different machined (unless your router sucks).
If your friend is trying to access your game using your PUBLIC IP address, have him try to get in with the INTERNAL address (192.168.xx.xx). If he's already using the internal, have him try the external.
You can't setup multiple DMZ hosts because that would be the same as two machines having the same IP address. As far as computers OUTSIDE your router are concerned, you have to have a seperate broadcast IP for EACH MACHINE.
I think you're stuck, man, just buy another DSL line
