You duplicate the entire <profile> tag, usuallly found in /etc/freeswitch/sip_profiles
A profile is binds to a single sip-ip and sip-port combination. You want to change the IP, port, or both on the new profile. rtp-ip *can* be the same, but is probably best matching the sip-ip (on a multihomed server you then know the media's taking the same working route that the signalling is).
Also on a real server with a static IP should I also remove auto-nat?
I don't use any nat stuff on a public IP. I have no ext-*-ip params set, which means they default to using the sip-ip and rtp-ip values.
I also start freeswitch with the -nonat flag (in the init.d script) which lets it start up much faster as it doesn't bother autodetecting your non-existent NAT router.
-Steve