My son is very excited to play your game.  He says it really needs to be played in coop mode.  I just burned an hour of my life trying to make Steam's family sharing plan work.  At this point, I have to conclude that it is impossible to play raft in local coop mode with each of us on our own computer unless we buy two copies of the game.  Am I right or am I wrong?  Please email the answer to jaycasper@gmail.com.

You have to buy 2 copies

Unfortunately, yes, two copies will be necessary.  I recommend finding a person willing to share their copy with you, since two games would be quite expensive, as it would be approximately 40$.