I think the game makers want you to buy one per PC. I have 3 PCs networked, and most games will require more then one copy to work over a LAN.
I don't have these games, so I don't know if this is your problem. If you are getting a message about CD keys, it may the the problem. If you are just not seeing the other PCs on your LAN, it may be a firwall blocking the game from using the network.
Gunner