Multiple computers in house

[The Jerk]

Hello all this is my first post. A friend of mine has two computers in his house and is using a cable modem and then a hub to split of to the computers.He has bought two copies of DFTD and would like to host on one computer while his daughter plays on his server on the other. It wont let him do this. They both however are able to play on my server at the same time.What happens is the computer hangs up when joining to his server.There are no firewalls.Has two nova world accounts with different names and passwords.His server hasnt any problems letting anyone else join. Just his other computer.Weird Stuff huh!Any help would be appreciated.

Thanks The Jerk