The following must be true:
Both PCs are logged into the same account.
The game is installed on both PCs.
One of PCs are in Steam offline mode.
Yes you can play on the offline computer in singleplayer, not multiplayer of any kind.
I do it all the time on my multiple PCs because i'm the only user, and because steam doesn't like that I try to use Evga PercisionX (a free overclocking program) on both PCs.