That setting actually tells you when multiple users log-in from the same IP.
Another variation I thought would be helpful is knowing if the same user logs in from multiple IPs during exact moments in time, or during a short period of time.
This might tell you if someone is sharing his log-in credentials (would be helpful for private/subscription boards).
To do the search you are asking for, you can go here:
Member Management / View/Edit Members / Advanced Search Options / Last Post IP (or Registration IP)
