I haven't tried it for a while, but I know from personal experience in the past it was easily possible to have two games running on the same account at once.
PC 1: Sign in to Steam, activate offline mode. Play game.
PC 2: Sign in to Steam on same account. Play game.
One account, one game purchased once, two people playing at the same time.
Yes, they have checks every so often (or did) for the offline computer, but in the scenarios given, this would work fine. As I say though, it's a while since I've done it, and then it was two computers on my same home network.
On the flip side though, what about this scenario:
Two people in the same house with their own consoles/steam machines. Both want a copy of Super Awesome New Game to play online at the same time with their friends. If we use something like Watch Dogs as a recent example, the Keeper of the Wallet is going to be forking out around £20 less every time they buy a game, and given how cheap older titles are on Steam (and stores that sell steamworks keys), buying the same game twice for two separate accounts isn't too bitter a pill to swallow.
Swings and roundabouts.