絶対タブインデックスを決定する限り、Javascriptでそれを行う方法はわかりません。ウィンドウは名前で識別できますが、それ以外は識別できません。
In your example of two tabs containing the same web page, you should be able to uniquely identify them by making them aware of each other. You'd need to use cookies for this. Essentially, when a page is loaded, it would check for a cookie that tells it about other instances of the page that are currently loaded, and make decisions accordingly.
In this scenario, your onload handler would check the cookies, and register the loading page. You'd also need an onunload handler to unset the cookie pertaining to the page being unloaded.
See Javascript communication between browser tabs/windows for more information on how to use cookies to communicate between windows with Javascript.