How to run multiple minecraft servers on one computer with different external ip address's?

"IF" you can get a second external IP address, you would need a second router, or a router that has some dual WAN inputs. Not that simple. That makes hosting the MC server on the same machine more difficult, since the router has to know which WAN input goes to which MC server.
That all said, it is true that two MC servers on one PC would need different ports on that machine. No way for a single PC to determine which MC server is being connected to, unless it is a different port.
One option would be to use one public IP, a different URL name for each MC server, with a different port assigned to the URL, and let the router do "PAT", port address translation. If the router can't do PAT, then just pass that second URL/port onto the second MC server directly. In basic terms, create/buy two URL names with different default ports assigned to them, and set up the MC servers to use those ports.