Load up the game mate then when on main menu release disk and stick it in the other pc going to be used in the LAN battle just to load up games and then you be away :)
But if you want to go online with both pcs i mean the internet lobby and gamespy server use and meet everyone and play on both pc's you need 2 different cd keys as when same cd key is logged on it will kick one off, this is to prevent copys of the sae disk being used, but its only like 5pounds now to buy Viking Invasion of Medieval or Shogun now anyhow :)