For those moment where you or your friend isn't available but the other wants to play without advancing further into the game than the other he could look for games in the browser that deals with content that the two of you have already done.
They can each start their own games to play single player, or co-op with other people, and the host could just load a particular save when the two who started that game could continue.
If you join a game, any required mods that you do not have can be transferred to your computer. IIRC that will be automatic, though presumably there will be a conformation at least with larger mods.
