Session is shared across all incognito tabs. Firefox behaves the same way. Even if you close any incognito tabs and then open a new one, it's still on the same session. You need to completely restart the browser (or explicitly clear history) to clear.
I don't have Chrome on the machine I'm using now so someone else needs to check it. But I vaguely remember running at work several sessions to the same service in separate incognito windows and maybe even in multiple tabs of the same window. Now that I think of it, I'm not sure about the latter scenario.