how to test multiple logins from single host ?

When logging in to a web server using watir or some other means
through rails test api, we would like to test things like multiple
users logged in as well as different functionality. The problem seems
to be that the website uses sessions to store various info and
multiple requests from the same host will appear to be as though one
session. Is there a way using some sort of IP spoofing or a different
port maybe to facilitate some means of making the server think there
are separate requests ?

You can use a different browser (Firefox and Mozilla, Mozilla and IE,
etc). They will get their own cookie and session ID