Testing simultaneous sessions...

Does anyone have any experience with testing (either manual or automated, preferably both) applications that require tracking multiple simultaneous sessions.

In my particular case, I have one "master" page, that will end up polling the server fairly often, along with 5+2n {n|-1<n<6} data entry pages.  I want to check, both by hand and via unit tests, that they are interacting properly.  However, I can't think of any good way to setup multiple sessions -- I might be able to get 2-3 by using different browsers (Opera, Safari, Firefox, etc) as I think they'll not clobber each other's sessions, even if on the same desktop.  But I don't know how I could setup the web test.