This is the source code of a discontinued mod for classic Serious Sam: The Second Encounter v1.07 that adds a lot of options for multiplayer games including coop mode support for singleplayer maps.
This source code is for the 1.9 version of the mod and it's based on the original Serious Sam SDK v1.07 released by Croteam in 2002.
Download the mod from here: https://dreamycecil.itch.io/advanced-mp
To compile the source code, you'll need to download the original Serious Sam SDK and copy these folders to it. The project files are for Microsoft Visual C++ 6.0.
You might need to setup extra things (such as the ENGINE_DIR directory).
Alternatively, you can put it in this SDK for the new Visual Studios: https://github.com/DreamyCecil/SeriousSam_SDK107
The source code of this mod is licensed under GNU GPL v2 (see LICENSE file).