Just as if you had bought a retail copy, you are allowed to install your copy of the game on however many computers you please. Of course, like the retail copy, you could only play one of these installations online. Not sure how you would go about getting a second set of CD Keys through GoG short of contacting them to see what arrangement can be made involving the purchase of another copy of the game.
Where I read it before, the question was something like "Can I install <multiplayergame> on my other computer so I can LAN with my friend?" and the answer was "Sure, though we ask that you only install it on computers you own". (I'll see if I can find the actual topic).
However, the requirement of unique CD keys prohibits this (some might even class this as DRM...). I was just wondering if anyone had already asked GoG about this scenario. I'll log a request for a key and ask them about it, report my findings back here :)
EDIT: Aha, here it is: http://www.gog.com/en/support/website_help/downloads_and_games
Can I install one game both on my laptop and desktop PC at home?
Yes. We do not limit the number of installations or reinstallations, as long as you install your purchased games on PCs in your household. So yeah, if you've got a render-farm in the basement, you might actually break the world record for the number of legal Fallout installations in one household. However, if you think about installing your game on a friend's PC or sharing it with others then please don't do it, okay?