Actually I'm not sure where we said that, I remember only stating that using Steam will be optional. But yes, truth is that you will be able to connect any and all of your accounts to your Steam account, mostly for convenience reasons. But Steam allows only one instance of any game that is launched from it.
Obviously you could still run one client from Steam and multiple others off-Steam, unless we explicitly prevent running multiple clients simultaneously.
As for the original question of this topic: purchasing multiple accounts on Steam probably won't be possible as Steam will generate an account for you, and once you have the game in your library, you won't be able to get it again. However, registering a new account on our website will still work like now.
edit: Yeah this wasn't about running multiple clients but I reckon that would have been the next question.