Halo: Campaign Evolved doesn't have split-screen on PC, but this mod fixes that if you're willing to look past some quirks

With a bit of work, you can enable split-screen co-op for Halo: Campaign Evolved on PC.
GameSpot

Halo: Campaign Evolved On PC Has A Missing Feature You Need A Mod To Access

PlayStation and Xbox players can already enjoy this feature, but Halo: Campaign Evolve's PC port needs a little outside help.

Mod activates split-screen support in Halo: Campaign Evolved PC that's just sitting there, unused

Come on, Halo Studios.