How to Log Into a Web App Simultaneously with Different Account?
CookieSwap to change users in just one firefox.
You can probably use Sandboxie to isolate each chrome instance by creating new sandboxes.
Use Firefox with the -no-remote option as many times you need
Problem is, you have to open several Firefox processes, the cookie swap extension seems better
You could use multiple browsers, or I believe the "Incognito Window" feature in Chrome would accomplish this, too.