well... i hate to break it too you...
but...
for LAN play... you only need one copy.
For ICO you need cd keys, yes, but for LANs... the game is very nice, and will let you install on your friends computer for LAN games.
so... basicly... next time its your friends birthday... you have a really easy present... a slip of paper with a cd key so that he can register his copy of sins with ICO, and he can have his own account, and play on ICO with the rest of us. (not to mention, you can set up passworded (or friends only) games... and you no longer have to be on a lan to play eachother.)
now... for the actualy answer to your question... would registering two copies cause wierd things to happen... I have no idea.