in simple steps :

1) download and instal on all workstation Trine 3 The Artifacts of Power
2) download and instal Sandboxie from official site http://www.sandboxie.com/index.php?DownloadSandboxie
2.1 ) if you use steam exit on all worksation
3) Run on first workstation Trine 3 The Artifacts of Power "THE CONFIG START and set window screen or if you like play full screen -> set no boarder
4) First player can stay in game. Second do the same thing but before launch Trine 3 The Artifacts of Power Run steam by Sandboxie -> login and choice your game
5) thats all enjoy your game 🙂