The game will support LAN and direct IP connections (it just isn't in the beta yet), so you do not need to use Steam at all other than to get the day 1 patch and future updates. You can install the game off the disk, which will reduce the download for the initial patch (though it may be a large patch, with some voiced lines being added). For your second PC you can copy the update from the first (unless you have a Mac/PC combo).
