Steam – Moving a game from game library to inventory on Steam

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).

Age of Empires Online is a free-to-play game with optional pay-to-play DLC. The All in Wonder bundle is a bundle of this DLC for the base game. This might seem like a minor distinction, but it's an important to understand what's happening here.

As with other free-to-play games in Steam, the base Age of Empires Online game will not appear in the "All games" list like a normal purchased game would, even if have purchased DLC for it. However, if you go to the steam store and search for the game there, you will be able to install the game client through Steam as normal.

As your paid-for DLC is associated with your Steam account it should still be available when you log into AoE Online through Steam.