Originally Posted by
Vespian
Not sure how Blizzard does it, but free games on Steam get added to your account and thus your key is linked to that account. On steam, you can give games to friends, but I'm not sure whether Blizzard offers those same options, so you might be looking at two copies of DII on your account without being able to transfer these.
On the other hand, the extra DII comes on USB, so it's likely that the key is a retail key in the set, which means you will be able to share it.
If I were you, I'd ask for clarification on the official forums, just to be sure.