It's been years since I tried it. Obviously back when Forza 2 came out.
Here's how it's done though:
The Setup:
You can configure multi-screen through the in-game Options menu “Network Cockpit” in many different ways. On any different sub-network you can configure up to 12 different “Network Cockpits”, each, however, must be assigned a different “channel” numbered 0-12. On each channel you can select up to 6 different views depending on your cockpit configuration. The “primary” view must be on the multi-screen “Host”, that is where the in-game HUD will appear. The primary view should be set to “Normal” or “Center”.
The available views per screen are:
“Normal” – the regular view you get when playing the game
“Center” – the center view of a three-screen configuration
“Right” – the right view of a three-screen configuration
“Left” – the left view of a three-screen configuration
“Replay” – a real time spectator view
“Rear” – a rear view mirror view
Some common view configurations:
3 screens: Center, Right and Left.
2 screens: Normal and Rear
2 screens: Normal and Replay
4 screens: Center, Right, Left and Rear
Set up your Xbox 360s and connect them to the hub or switch. Then connect the hub or switch to the rest of your network and launch Forza Motorsport 2 on all consoles. Your profile and Xbox Live account should be located on the primary screen.