Restoring co-op/split-screen is one of the many features H2.5 would enable, a project that is still being worked on.
The original maps do not support co-op play very well because some maps are missing a secondary player spawn.
Whereas one issue with split-screen is the resolutions/aspect ratios are based on OG Xbox settings, which means a low res of 480 by whatever. Another issue is the inputs, Himanshu thinks he might be able to fix that, I have sent him the files so that he can compile and test his own custom mainmenu (H2.5) content.
I have many goals for the PC iterations of Halo, its a damn shame that we do not have any proper Halo co-op yet, but it will happen, eventually.