How do you play 2 player on Halo 3 custom games on the same system?

[ANA]

and in all the other lobbys?

 

[A$H]

Just turn on the other xbox 360 controller...you may have to do the wireless scan to find the controller.
It should then just clone your player name into the lobby with a (2) at the end. when the game starts it will be split screen.
This works with up to 4 players |(3) & (4)| but you get quarter screen each then.

 

[perfecsaiyan]

Yes you can do 2 players on the same system.