Because the game doesn't support multiple save points, I need to make a backup of my progress so my partner can play too.
I've found the save data in C:\Program Files (x86)\Steam\userdata\XXXX\8870\remote\savedata but making a copy, and later restoring it, doesn't work. I've lost all my progress! I have disabled Steam cloud support in the in-game options, but my original save game is never recognized and I can only start a new game.