You can program exceptions on systems, for example I could put that a character named Plague, bypass the IP check on the Dungeons. We can also add another check on the network card or the HWID of the hard drive, so that if you are on this white list and you use both accounts on 1 computer, you can't log, but if you use two computers, you can.
Again don't do a bandage in your head if is not broken there is always a solution for everything.