I did see a post be someone using different ports while on the same IP address but that was more in passing than as an instruction on how to do it. what you actually do is connect 1 at a time log into the web interface and change the IP manually.  some suggest using DHCP but the IP address is random so it makes accessing the web interface a bit hard if you don't know for sure what the IP address is

anyway the guide you are after is

https://bitcointalk.org/index.php?topic=702653