Yes, you'll be able to play together with a Steam and GOG version, as well as PC/Mac, through LAN or Direct IP (Larian is looking into GOG Galaxy, as well, but that may be awhile).
If you get the CE on Steam then the release will be DRM free, so if it wasn't for the PC/Mac difference you could install/update the game on your computer and copy it over to your wife's computer (you'd have 2 keys and 2 copies of the game).
GOG may have a CE version with a couple copies of the game, as well, but they have not announced any pre-orders, and at this point it seems it will just show up on release (or maybe pre-orders the weekend before).
