Steam – 2 Steam Accounts W/ Different Games

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).

The Steam executable (Steam.exe on Windows) accepts a number of command line parameters; you could try taking advantage of those.

Specifically, this one, which starts Steam and immediately attempts to log in with the given credentials:

-login %u %p

This logs into Steam with the specified Username and Password combination. Replace %u with the username, and %p with the password you want to login with (Steam must be off for this to work).

And, since this doesn't work while Steam is running:

-shutdown

This shuts down (exits) Steam.