in simple steps :

1) download and instal on all workstation Terraria
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 Terraria them go to options > video 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 Terraria Run steam by Sandboxie -> login and choice your game
4.1) You can set local game or steam game or just single if you dont like your friends 🙂
5) thats all enjoy your game 🙂