play with two people by playing split screen
yes
Yes, even with one or two players.
You can have more you can have up to 4
Campaign can be played by up to two players locally, and four players over LIVE or system link.
yes sign in two controllers and you can go on any game type except theater
You can play two players on halo 3 with two controllers
you must have several achievements from HALO 3 and HALO ODST. Along with certain achievements from HALO REACH. Then you will unlock the two helmets
There are now two, one from halo reach and one from halo way-point. The first is obtained by obtaining a bull-true medal on an on-line game in halo reach, and the other for earning enough achievements to be rank 50 on halo way-point.
halo wars, halo reach, halo CE, halo 2, halo 3
It is not possible
Actually, Halo Reach will allow 4 players to play through the campaign, split screen or through Xbox Live. It's perfectly possible to play with only two or three players as well. As a side note, you can play split screen with your buddy and play with two others on Xbox Live, provided both of you have a Gold Membership.
Halo 3: ODST is the newest of the two, but not the newest Halo game.