How do I forward the SAME port to 2 computers.
Generally you do not. The unique target is a combination of IP:port. You can not use two identical values while having each of those unique.
HTTP is an exception though, it uses a triple {IP:port:hostname in protocol}. You can formward to a single computer running a webserver/proxy. And then based on that forward is indirectly to the destination host.
Note that this is the same as Ignacio already said, but with more background.
Also note that this does not depened on a router or a brnad of router.