You should be able to run sessions for two different users on the same machine, at least running IE on Windows. Start your browser and log in as user X. Start a second browser, but make sure you run it as a separate instance of IE: launch the IE application again by clicking on its icon. Log in as user Y. The two browser instances don't their share non-persistent cookies, and will in fact be different user sessions in OpenACS. I tested it with a little older version of OACS, and it works.
Note that this doesn't work if you choose File > New > Window in IE - then you'll get another window, but it's the same instance of IE with the same non-persistent cookies. Also, if you check the box to "Remember this login", your login is stored in a persistent cookie, and the different browser instances can interfere with each other (the second instance would overwrite the persistent cookie set by the first, I think).
