in simple steps :

1) download and instal on all workstation Trackmania Turbo
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 Trackmania Turbo them go to options >(mowe mouse to top screen and menu will appear/or click left ALT+ENTER) 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 Trackmania Turbo Run steam by Sandboxie -> login and choice your game
5) thats all enjoy your game 🙂