Well, strictly speaking, you don't
need to, but it's considerate to buy two copies if you have two people playing at the same time. But the game doesn't have any DRM built into it, as far as I know.
Back when I was first starting, rather then just play a 3 hour demo, my friend just had me install it with his cd key, so that way we were able to play most of a campaign together, and then I decided that I quite liked it, and so I got my own copies of both games. But the game is still installed using his keys..... Which doesn't make the slightest bit of difference to anyone. The game works fine, Arcen has my money, I have my keys, etc.
So there isn't any
need to have more then one key, but it is nice to support such an awesome game company. If it's always the same friend, maybe you could talk him into buying his own copy, and then you wouldn't need to fret about it.