It would obviously be against the terms and conditions of Steam. But if you really wanted to do this it could up to be quite an administrative headache. AFAIK Steam now sends out email confirmations (similar to how online banking works with one time passwords) if they detect that you are connecting from a new PC or new IP. You guys would have to agree on a shared email account as well, or have one guy play the master role. Which could possibly leave the slave stranded for a few hours or a day.
Steam can also be very temperamental when it comes to offline mode. I have never really found offline mode to be reliable enough to be 100% sure that I would be able to game without an Internet connection.
