So, I've never used Steam before firing up F:NV a couple of days ago. Seems to work fine, no problem. But when my wife wants to play, it seems we cannot have the client open on two different computers. For us both to play at the same time do we need to have two separate Steam accounts - and buy the game twice?

Thanks

You can have the one game on many computers, but you can NOT have the game open at once on many different computers. The only legal way that you can both play the game at the same time is to both purchase copies. The game can only be associated with one Steam account. I'm not sure what functional purpose this serves but if you don't want to tell your wife to shell out for another game I suggest just rationing your NV hours so you can still both play.
It's rather irritating and my boyfriend and I who live together were also rather irritated when we first found out as it seems a bit ridiculous to not even allow the sharing of the game on one other account. I guess that's how companies make trillions.

The developers have back-handedly mentioned that Steam in offline mode doesn't connect to the internet and therefore can't tell if the account is already in use.

Yup. Doesn't connect in offline mode. You can physically disconnect your internet and still play. So...yeah...offline mode. Go for it.

This didn't seem to work for me? I have started the game in offline mode and if my boyfriend then tried to play the game, I would get signed out and no longer be able to play the game. Even turning the modem off didn't work.

Are your computers networked together (so they can both access the internet?) Maybe something to do with that?
eg, even if not connected to net, maybe Steam client can see over your network to the different machines? I don't know if that's even technically possible, just a wild theory ...

Addition: I asked my husband if that was even possible and he said "yes, if the client is set to communicate over a network like that."
So I'm not gonna say Steam's client is or isn't, but try unplugging your network cable between computers (or turning off your wireless if you're using that) and see if it works?

He ended up having to get his own copy ^_^

*edit* Where are my manners? Thank you for the suggestions anyway!

One account - one game, otherwise you would be able to have lots of different people playing the game by sharing an account.