You buy a license for one person to play the game. Not you and anyone you let play it.
If you both play the game, buy two copies. That's the license.
If only she plays the game, get her her own Steam account and buy it there - you CAN play two games at the same time on separate accounts.
meh that is bullshit. If he had two retail version this wouldn't' be an issue.
Sure, you will say, "then buy a retail version with a different license setup" but as PC gaming shifts more and more towards steam style distribution, consumers need to fight back to be able to use their purchases legitimately. Of course it comes down to the definition of legitimate, but I'd prefer the
paying consumer define that term, and IMO he is using it legitimately.
But yeah to the OP: I'd try to set it up in offline mode, disconnect your internet so steam is truly offline, and see if it works. If you paid for both games I personally think its restrictive to say that your GF cannot use your account to play at the same time when you are both together. Imo there is a HUGE difference in intent between that and sharing an account with many friends so you can all share each other's games simultaneously (Which is probably why you can only have a single unique log on at any time). It may be hard to detect and formalize that, but i think the right exists. Just like someone said, I can play WOW and SC2 at the same time. I can also buy retail versions of games and do that; hell I can do that and have both copies be 50 miles apart, and it wouldn't matter... No reason that I shouldn't be able to with other things as well. We need to carefully think about the causes of piracy...and while there are many causes, IMO this would be another reason to engage in piracy (ie: you paid for both, but your GF can't play, so you get a pirated copy on her PC so she can use the game that you bought but don't play).