A receives a notification that they are signed on at B's location, and A gets logged out. I leave my access open at home, and when I go to log in at work, there's always a slight delay - then, when I get home, there's a notice about the sign-out & log-in, and I have to log in at home.
As with mud, life, too, slides by.
I just logged into both my facebook and gmail using my phone- while having those same sites up in the browser on my laptop.
The facebook I could navigate with either device without detecting the others presence. The gmail, while not real-time, did notice some changes. The mobile version noted the changes (once refreshed) while the laptop took longer to see the changes, if at all. And didn't get kicked off the account at any time on either site.