Here's what you need to do:
1. Start up multi-player on one of the computers.
2. Start up a Lan game (doesnt matter which mod)
3. Click Advance Options in the lower right hand corner
4. Change the Server Port # to a diffrent Number (ie: Your Fathers is 2212 yours is 2212 change yours to 2213 this is just a example some numbers are way diffrent)
5. Click ok
This should save the information for you and allow both you and your father to play the game.
That is probably what the problem is, I hope this solves it for you. Good luck and see you in game.
Sakura-Chan