you can pretty much just have fun and play normally, but there's another cabin added to the farm depending on how many players are playing. You can choose an option of shared money or separate money. You can only play in the coop if the host is actively playing. If you want to marry your gf in game, you can make a wedding ring with the sewing machine in Emily's house. The rest is pretty much like the single player game.
Co-op play is fun and you can share tasks, like one runs to town to grab seeds while the other gets the garden space ready. A huge downfall is that the in game time does not pause, meaning even if you have menu up, its still ticking by. There is a keyboard prompt to pause on/off though.... in the text chat /pause and /resume