Hi Hilton,
Sorry - but you're fighting the tide here. Some background info here will be helpful.
Firstly, browsers give no indication which tab a request is made from. From the servers point of view, there are no "tabs". There's just a browser making requests. So it's not possibly to have two users logged in from the same browser.
Secondly, browsers behave differently when it comes to multiple "instances". IE, for example treats each instance differently, and each can have a unique session id. But other browsers (like FireFox) are actually all one instance, even if you "run" it multiple times.
So this approach for testing multiple users is flawed. You'll need to test from multiple machines. Or you can run one instance of each browser (IE, FF, Chrome, Safari, Opera) on the same machine.
Cheers
Bruce