You can't have multiple IPs assigned to the same forwarded port. It's a logic issue, it just doesn't make sense and would be impossible to use correctly. What you can do is use Port Triggering.
Port Triggering is essentially a dynamic port forwarding. What you do is setup a "trigger" port and causes the router to forward a certain port to that client.
eg. Lets say GameX needs to connect to a server on port 12345 and you need port 23456 forwarded to you for some reason. You set the trigger port to 12345 and when ANY client connects to that port on another machine, it automatically sets up port 23456 to be forwarded to that client.
As you can only have one IP forwarded per port, you still have the issue of Wii or xBox, but it'll choose whichever one is currently being used.