There are a few variations of this mod, but the standard mod is the most common.

Navigate to your GameData\addon\ folder. Inside the v1.3 folder, look for a file called SplitScreenConfig.txt (or use the launcher utility).

: This dedicated tool is designed specifically for the 2005 PC version. It includes a Split Screen GUI to configure play sessions and uses a pre-created profile to provide controls nearly identical to the original console versions.