Steam – Installing Games to Multiple Locations

As far as I know, you can't do that. If one of you wanted to play single player, then you could possibly set one computer to Offline Mode before starting the game, and then the other person should be able to log in on the other account and play multiplayer, but both of you being online at the same time should not be possible.

Keep in mind, this is technically not allowed. Steam's agreement says you're not allowed to share your account or your games, that includes your fiancee. I don't think they actually do anything about it as long as you're not talking about it on steam chat or their forums. However, you still risk losing your account and all your games when bypassing their rules.

EDIT: As of mid 2014, Steam has released Steam Family Sharing, which enables the sharing of Steam libraries. However, this system does not work on a per-title basis (only during the beta), but rather - If one title is being played, all other titles cannot be played (unless it's the same user that's using the library that is launching the second/whatever-is-after-first program).

You cannot install a Steam game offline. It needs to connect to the Steam servers the very first time to confirm you own the game.

If you do not want to redownload the game, go online, click install, let it go as far as where it starts downloading, close Steam completely and copy the files from the SteamApps folder on another machine, then launch Steam again. However, this process still requires an internet connection on first play.