I can't promise you anything, but after reading the First Strike the halo novel that takes place after Halo I found a few very interesting things. Remember how Master Cheif was the last Spartan left, not quite.
Check out the E3 video and watch as the soldiers are suiting up.
http://files.mythica.org/halo/halo2_...demo_hires.wmv
Yes that is two Spartans on the same screen
Here's how thats possible- Reach was not completely destroyed- several Spartans were trapped inside the fallback position. At the end of the book only 4 Spartans are still alive- this could be evidence that 4 player co-op might be put into the game because there is Master Cheif of course. Linda the sniper. Will and Fred, two other guys.